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Today, we're talking about
immunology as you may recall.

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Let's roll.
Yes, let's roll.

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And by the way, admire
your little finger,

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because most of the planet
isn't as gifted as you are.

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Not everyone was guaranteed
so many brains as you and I,

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seriously, have been fortunate
enough to have been granted.

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Let me just finish up what we
were talking about last time,

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which was the
cell cycle.

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You may recall that we talked about
oncogenes. We talked about Ras and

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the Ras oncoprotein, which
sends out mytogenic signals.

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And, what it does, just to
summarize what we were talking about

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is it pushes cells from the
beginning of the G1 phase of the

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cell cycle up to a decision
point in the life of the cell.

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It's called the restriction point.
And here, just by way of review,

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the cell has to decide whether or
not it's going to commit itself,

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essentially irreversibly, to go
through the rest of the cell cycle,

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or whether it'll stay in G1, and
may even retreat from the cell cycle

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into G0. And Ras pushes
this decision forward.

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The retinoblastoma protein, which
we talked about the last time,

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stands as the guardian of the
gate right here, the RB protein.

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And, the retinoblastoma protein
holds this gate shut unless and

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until certain preconditions
have been satisfied,

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on which occasion the retinoblastoma
protein opens up the restriction

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point gate, and allows the cell
to go through the rest of the cell

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cycle. And now, interviewed
from this perspective,

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we can begin to understand
how hyperactivity of Ras,

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and how the inactivation of this
RB, this retinoblastoma, tumor

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suppressor protein have such
disruptive destabilizing effects on

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the proliferative
controls of the cell.

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Keep in mind, this is a negative
actor on cell proliferation.

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It's a tumor suppressor gene which
must be inactivated in many cancers.

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This is a proto-oncogene, or
an oncogene, which must become

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hyperactivated. Now, I
want to move from that into

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the topic of today, and
that's the whole issue of

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immunity. And, much of
our immunity comes from

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understanding the way we deal
with what viral infections.

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The fact is, just to cite
one arbitrary viral infection,

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relatively few people die from viral
infections these days because we can

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become immunized against them.
The first immunizations already

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began in the late 18th century,
believe it or not, when a physician

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in England called Edward Jenner
first noticed that women who had

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worked as milkmaids milking cows,
and who got a disease called cowpox

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from milking the cows seemed to be
immune to the disease of smallpox

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which was by that time realized
to be spreading in epidemic,

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so a highly infectious agent, and
one which actually killed quite

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a few people. And Jenner
intuited correctly,

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in retrospect, that the experience
of these milkmaids and their

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exposure to cowpox exposure
somehow protected them,

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gave them indeed lifelong protection
from subsequent smallpox infection.

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Subsequently to that, the
sores from the cowpox infection

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were scraped, and the exidate, the
fluid, was scratched into wounds

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of people in order to immunize them.
And so, immunization them. And so,

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immunization already began in the
1790's, taking a sore from the skin

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of a cowpox infected patient,
injecting that into the skin of

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somebody who required immunization,
and as a consequence, hoping that

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this would confer in
them lifelong protection.

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In some cases, actually
the individuals who were

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infected in that way actually came
down with smallpox or some virulent

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form of this cowpox, but
in most other cases these

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individuals actually
acquired a lifelong immunity.

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In fact, the very word vaccine,
which was used already at the time,

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comes from the Latin word
vaccinus which means a cow.

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And, in the north side of Cambridge
Common there's the Benjamin

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Waterhouse which is still there.
He was the first physician to

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introduce smallpox vaccination into
this country already in the end of

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the 18th century. If we
fast forward to a situation

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like poliovirus, we have
situations in this country

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where in the 1930's-1940's, there
were epidemics of poliovirus.

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If one began to examine who
was susceptible and who wasn't,

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it was clear that children who
were, for example, born and raised in

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middle and upper-middle class houses
were very susceptible for much of

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their lives. For example, in
southern California there were

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dramatic examples, whereas
children who grew up across

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the border in,
let's say, Tijuana,

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Mexico, rarely came down
with paralytic polio.

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Poliovirus, as one soon learned,
was a virus which infects not only

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the gastrointestinal tract, and
creates a form of mild diarrhea,

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but it may be one out of 100 persons
the virus escapes from the GI tract,

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from the gastrointestinal tract,
invades into the central nervous

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system, and actually creates
debilitating paralysis,

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most of which is not healed.
And some people have lifelong

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paralysis. Other
individuals whose paralytic

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paralysis goes away,
actually when they grow older,

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30, 40, 50 years later, they
begin once again to experience the

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paralytic symptoms that arose as
a consequence of their childhood

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infection. And at this time, one
began to try to figure out why

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children living in Tijuana,
Mexico rarely came down with

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poliovirus infections, whereas
those who grew up up north

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like, say, in southern
California did indeed do so.

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And one came to the conclusion that
the children growing up in Tijuana,

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Mexico were frequently exposed very
early in their life to contaminated

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water, sewage contaminated water,
and they as a consequence acquired a

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lifelong immunity without getting
sick, whereas children who grew up

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in rather sterile conditions further
north never had any exposure to the

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virus. And when it hit them as
young adults or as teenagers,

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it created devastating effects.
And this indicates, once again,

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that somehow one's exposure
to an infectious agent,

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historical exposure has a dramatic
effect on one's susceptibility

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to a virus. And in
the case of poliovirus,

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we're dealing here with an agent
which is much simpler than the

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retroviruses we talked
about last time like RSV.

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Poliovirus also has a
single stranded RNA genome,

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and that's encapsidated
in a proteinaceous coat,

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which is made up only of viral
proteins, so it's very simple single

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stranded RNA proteinaceous coat.
The single stranded RNA is actually

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polyadenylated at the three
prime end. So, it can serve as a

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messenger RNA. It's of
the same polarity that has

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a plus polarity, which
means that it can be

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translated immediately. I'm
distinguishing that from the

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other flavor of single stranded RNA
which could be of the complementary

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strand, which could exist because
one could make double stranded RNA

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which obviously
cannot be translated.

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So, this can serve as a messenger
RNA, and just as an aside,

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the way that poliovirus replicates
is totally different from that of

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retroviruses. What happens is that
poliovirus makes its own polymerase.

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You go from single stranded
RNA to double stranded RNA,

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i.e. the complementary copy is
made, and that in turn is used as a

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template for making more single
stranded RNA, progeny RNA.

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And note here that there's no
DNA at all involved. In fact,

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poliovirus can grow inside a
cell that has been deprived of its

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nucleus. Moreover, if one
stops nuclear DNA dependent

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transcription, that
has no effect on this.

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But you'll notice correctly that
these kind of steps here involved

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RNA dependent RNA polymerases.
And in the normal physiology of a

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cell, that such a polymerase,
such an enzyme never operates.

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And therefore, poliovirus must
make among its other proteins an RNA

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dependent RNA polymerase that
can mediate these steps: making a

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complementary copy of this strand,
to get the complementary strand, and

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then making progeny
plus strand RNAs.

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Again, it has a very simple capsid
of several viral proteins which wrap

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it up. Now, if one wants to assess
the potency of poliovirus RNA,

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one can grow it in a Petri dish.
One way of figuring out how much

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poliovirus RNA is in a solution is
to make a monolayer of cells like

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this, and then take various
dilutions of a virus stalk.

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And when one talks about a virus
stalk, one talks about a solution of

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virus particles. One
can't really see them.

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And even if one could see them
under the electron microscope,

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one wouldn't really know what
fraction of the ones you were seeing

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were actually biologically
competent. But most interestingly,

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you can take a solution
of poliovirus particles,

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place it on a monolayer of
cells here, which can be infected

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by poliovirus. In contrast
to what I told you last

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time about RSV, where an
infected cell can tolerate

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the continued presence of a
viral infection without dying,

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poliovirus is a highly cytopathic
virus, and by that I mean it

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replicates inside cells and it
kills them during the course of

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replicating inside these cells.
So, here what one has is just a

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poliovirus will infect a cell here,
and then it will begin to spread

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from that cell to neighboring
cells in the surrounds.

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And in so doing it will create,
it will erode a hole right here in

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the monolayer whose presence, a
so-called plaque, signifies the

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fact that there was an initially
infecting virus particle there that

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spreads centrifugally, that
spread outward, from the

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initially infected cell. In
detail, if you want to do this

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experiment really nicely, what
you do after you infect the

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initial monolayer of cells,
which I show here in section,

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is you put on a layer of agarose,
or something, above the infected

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monolayer, and that ensures
that if there's any viral spread,

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it will be from cell to cell spread
in a certain neighborhood rather

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than virus particles getting into
solution and swimming around all

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over the plate and affecting
cells helter-skelter.

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So here, we might want to infect
an initial cell right here in the

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monolayer, and now this agarose
will confine the subsequent spread of

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progeny virus to adjacent cells
in the area, again eroding,

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ultimately, a hole here
which we call a plaque.

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And if we count the number of
plaques, that will tell us how many

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biologically active virus particles
there were in the initial solution

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that was previously applied to that
monolayer culture of the susceptible

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cells. One could use such monolayer
cultures, in fact, to propagate

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poliovirus. And one
can take the poliovirus

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coming out of those monolayer
cultures, and actually use them to

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inject into human beings in order
to vaccinate them to confer on them

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immunity. But if you do that
with wild type poliovirus,

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then what will happen is that you
may inadvertently give that person

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whom you're attempting to immunize
a nasty poliovirus infection which

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could paralyze them,
might even kill them.

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When I was growing up,
poliovirus infections were much

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dreaded because we knew of people
who were being kept alive in iron

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lungs that breathe for them because
their autonomic nervous system had

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been destroyed by a
poliovirus infection.

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So, what happens is therefore if
one wishes to immunize somebody,

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if one wishes to vaccinate
them against poliovirus,

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one needs to inject them
with an attenuated virus,

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that is, a virus whose
ability to create disease,

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whose pathogenic abilities,
pathogenic, remember, refers to the

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ability to create disease. You
want to use an attenuated virus,

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which might be able to go into
the body. It might be able to evoke

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immunity, but it will not
create a major disease response.

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And this was dealt with
in two different ways.

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Jonas Salk, one of the pioneers of
creating a poliovirus vaccine took

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the poliovirus particles that have
been produced by culturing them on

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monolayers of monkey kidney cells,
and he treated the poliovirus

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briefly with a little
bit of formaldehyde.

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And formaldehyde, as you
may know, reacts with the

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amine groups of the ribonucleosides,
of the purines and pyrimidines, and

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therefore kills the virus. And
that kill virus, having been

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treated briefly with the
formaldehyde, the virus particle is

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still essentially intact, was
then injected into individuals.

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Alternatively,

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his deadly rival, because
they two hated each other's

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guts, Sabin decided on another
strategy for making a vaccine stalk.

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And that is, he took poliovirus
which had been passage from one cell

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culture to the next over
extended periods of time,

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over several years. So, it
had been passage in vitro.

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When we say in vitro, in this
context we mean poliovirus had been

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taken from one plate, put
into another plate of cells,

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taken to another plate of cells.
In vitro here implies in culture

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rather than the other alternative.
In vivo means that the virus is

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being passaged in a living organism.
And as it turns out, when you

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passage poliovirus in vitro from
passing it just from one culture of

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cells to the next, infecting
one after another in a

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serial or sequential fashion,
then the virus is selected for its

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ability to proliferate in
the cultured cells in vitro.

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But it gradually loses its ability
to create disease in vivo in that

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there is no Darwinian selection to
favor its disease causing ability.

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And consequently, virus that
Saben used wasn't inactivated with

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formaldehyde. It was a simply
attenuated because of extensive in

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vitro propagation. And
when he injected that into

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young people, very rarely did
they ever come down with poliovirus

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infection, and the virus was able
to replicate to some extent in their

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bodies without causing disease,
and to create lifelong immunity.

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I.e. such an individual was
protected from subsequent poliovirus

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00:15:41,000 --> 00:15:46,000
infection ever again.
And, as it turned out,

224
00:15:46,000 --> 00:15:50,000
this was a very nice way of
protecting because it also turned

225
00:15:50,000 --> 00:15:55,000
out the monkey kidney cells that
were used to propagate the virus

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00:15:55,000 --> 00:15:59,000
also contained certain
monkey viruses. For example,

227
00:15:59,000 --> 00:16:04,000
it came out after years that
there was a DNA tumor virus.

228
00:16:04,000 --> 00:16:07,000
We've been talking about
RNA tumor viruses until now,

229
00:16:07,000 --> 00:16:11,000
but there's a DNA tumor virus called
SV40, which is a very potent tumor

230
00:16:11,000 --> 00:16:15,000
virus in hamsters,
and mice, and rats.

231
00:16:15,000 --> 00:16:18,000
And it turns out that SV40 lingered.
It lurked in monkey kidney cell

232
00:16:18,000 --> 00:16:22,000
cultures. And,
nobody knew about it.

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00:16:22,000 --> 00:16:26,000
Whenever you put poliovirus
into many of these cultures,

234
00:16:26,000 --> 00:16:30,000
not only did poliovirus replicate,
but all of a sudden so did SV40.

235
00:16:30,000 --> 00:16:34,000
And therefore, many of
the stalks of poliovirus

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00:16:34,000 --> 00:16:38,000
that were used to inject people like
myself, I lived around the corner in

237
00:16:38,000 --> 00:16:42,000
Pittsburgh from Jonas Salk, and
the children in the Pittsburgh

238
00:16:42,000 --> 00:16:46,000
public schools were his first guinea
pigs. Many of us were exposed to

239
00:16:46,000 --> 00:16:50,000
poliovirus stalk which actually
had more SV40 particles in it than

240
00:16:50,000 --> 00:16:54,000
poliovirus particles because of
this inadvertent contamination.

241
00:16:54,000 --> 00:16:58,000
There probably were between 30 and
40 million people who were immunized

242
00:16:58,000 --> 00:17:03,000
with poliovirus and
inadvertently with SV40 virus.

243
00:17:03,000 --> 00:17:07,000
And one is that epidemiology in
the subsequent years to figure out

244
00:17:07,000 --> 00:17:11,000
whether that had led to any
increased rate of cancer because it

245
00:17:11,000 --> 00:17:15,000
could well have. There
could have been an epidemic

246
00:17:15,000 --> 00:17:19,000
of cancer in this country,
millions of people affected because

247
00:17:19,000 --> 00:17:23,000
of this unknown, and at
the time almost unknowable

248
00:17:23,000 --> 00:17:27,000
contaminating virus. As it
turns out, people who were

249
00:17:27,000 --> 00:17:31,000
infected with SV40 inadvertently
have no higher rates of cancer than

250
00:17:31,000 --> 00:17:35,000
anybody else even though this virus
is a very potently tumorogenic virus

251
00:17:35,000 --> 00:17:39,000
like RSV, but doing so in this
case in rodent cells and ostensibly

252
00:17:39,000 --> 00:17:43,000
not in humans. So,
public health measures can

253
00:17:43,000 --> 00:17:47,000
sometimes have unforeseen side
effects or consequences that nobody

254
00:17:47,000 --> 00:17:51,000
anticipates ahead of time.
Now, getting back to the whole

255
00:17:51,000 --> 00:17:56,000
strategy of poliovirus infection,
this raises the issue of what it was

256
00:17:56,000 --> 00:18:00,000
in the poliovirus that was able
to evoke the subsequent lifelong

257
00:18:00,000 --> 00:18:05,000
immunity. And in telling you
that, I mentioned the following.

258
00:18:05,000 --> 00:18:09,000
You can make antiserum or you can
make serum from an individual who's

259
00:18:09,000 --> 00:18:13,000
been immunized with poliovirus.
And, recollect, we mentioned this

260
00:18:13,000 --> 00:18:17,000
before, the way to make serum
is to allow blood to clot.

261
00:18:17,000 --> 00:18:21,000
The red cells and the platelets
aggregate in the bottom,

262
00:18:21,000 --> 00:18:25,000
and what remains on top is simply
serum. You can spin out all the

263
00:18:25,000 --> 00:18:30,000
residual cells and so all you get
is sort of a straw colored fluid.

264
00:18:30,000 --> 00:18:34,000
And that serum from an individual
who's been immunized with poliovirus

265
00:18:34,000 --> 00:18:38,000
stalk can actually be used to,
you can add it to the poliovirus

266
00:18:38,000 --> 00:18:42,000
stalk prior to adding these
virus particles to the Petri dish.

267
00:18:42,000 --> 00:18:46,000
So, here's the Petri dish as before,
and what you find is that if you add

268
00:18:46,000 --> 00:18:50,000
serum from an individual who's
been immunized to a solution of

269
00:18:50,000 --> 00:18:54,000
poliovirus particles, and
then you take that mixture and

270
00:18:54,000 --> 00:18:58,000
put them on the plate, you
no longer get any of these

271
00:18:58,000 --> 00:19:03,000
plaques that I
referred to before.

272
00:19:03,000 --> 00:19:07,000
And that indicates that within the
serum, there is some kind of factor

273
00:19:07,000 --> 00:19:12,000
which is neutralizing the
infectivity of the polio virus

274
00:19:12,000 --> 00:19:16,000
particles, i.e. the poliovirus
particles are in some

275
00:19:16,000 --> 00:19:21,000
way prevented from creating an
infection, and their plaque forming

276
00:19:21,000 --> 00:19:26,000
ability, their ability corrode
these holes in the monolayers of

277
00:19:26,000 --> 00:19:31,000
subsequently infected
cultures is now compromised.

278
00:19:31,000 --> 00:19:35,000
And such activity suggests that such
an individual has actually antiserum,

279
00:19:35,000 --> 00:19:39,000
i.e. some kind of reactivity which
prevents the poliovirus particle

280
00:19:39,000 --> 00:19:44,000
from doing its thing. In
fact, I can tell you that such

281
00:19:44,000 --> 00:19:48,000
antiserum or such neutralizing
antiserum can be achieved in a

282
00:19:48,000 --> 00:19:53,000
number of ways. One way
to do it is just to take

283
00:19:53,000 --> 00:19:57,000
the protein capsid, the
outer coat of the poliovirus

284
00:19:57,000 --> 00:20:02,000
particle that I described before
and inject that into an individual.

285
00:20:02,000 --> 00:20:06,000
And that will on its own already
evoke a measure of antiviral

286
00:20:06,000 --> 00:20:10,000
immunity. It will evoke a
concentration of antiserum in the

287
00:20:10,000 --> 00:20:15,000
blood of a patient. Or, if
you want to do it in a very

288
00:20:15,000 --> 00:20:19,000
much more modern way, you can
sequence the proteins of the

289
00:20:19,000 --> 00:20:24,000
poliovirus and determine
the amino acid sequence.

290
00:20:24,000 --> 00:20:28,000
And when you do that, you
can synthesize through organic

291
00:20:28,000 --> 00:20:33,000
synthesis oligopeptides of
10 or 20 amino acids long.

292
00:20:33,000 --> 00:20:36,000
And these oligopeptides reflect
different parts of the poliovirus

293
00:20:36,000 --> 00:20:39,000
coat protein. So, if here's
the sequence of one of the

294
00:20:39,000 --> 00:20:43,000
capsid proteins, recall
that capsid refers to the

295
00:20:43,000 --> 00:20:46,000
coat that's shielding or
protecting in this case poliovirus,

296
00:20:46,000 --> 00:20:50,000
the single stranded viral RNA genome.
So, if here's one of the capsid

297
00:20:50,000 --> 00:20:53,000
proteins, what you can do is to
figure out the amino acid sequence

298
00:20:53,000 --> 00:20:57,000
of this segment of the capsid
protein, and then rather than

299
00:20:57,000 --> 00:21:00,000
cutting it out from the capsid
protein, you just synthesize it by

300
00:21:00,000 --> 00:21:04,000
organic synthesis, ten or
twenty amino acid residues

301
00:21:04,000 --> 00:21:08,000
long. And when
you inject that

302
00:21:08,000 --> 00:21:12,000
oligopeptides into an individual,
that may also evoke some kind of

303
00:21:12,000 --> 00:21:16,000
neutralizing antibody response.
In fact, as you might correctly

304
00:21:16,000 --> 00:21:21,000
imagine, if we can look in more
detail at the capsid structure,

305
00:21:21,000 --> 00:21:25,000
I'm just going to draw it
in a very primitive way here,

306
00:21:25,000 --> 00:21:29,000
it's obviously much more complicated.
But if this is the capsid here,

307
00:21:29,000 --> 00:21:34,000
it has an inside and an outside.
And the antibodies that you make

308
00:21:34,000 --> 00:21:39,000
against an oligopeptide that is
sticking out here on the outside

309
00:21:39,000 --> 00:21:43,000
will be neutralizing, i.e.
the antiserum will be able to

310
00:21:43,000 --> 00:21:48,000
recognize the outside of the virus
particle and somehow inactivate it.

311
00:21:48,000 --> 00:21:53,000
But if you were to use an
oligopeptide antigen from over here,

312
00:21:53,000 --> 00:21:57,000
which might be a part of the
capsid protein that is deep inside,

313
00:21:57,000 --> 00:22:02,000
well even though you might have a
good reactivity with oligopeptide it

314
00:22:02,000 --> 00:22:07,000
would not be neutralizing. And
that suggests the notion that

315
00:22:07,000 --> 00:22:11,000
whatever is present in the antiserum
is recognizing some external portion

316
00:22:11,000 --> 00:22:16,000
of this protein which is sticking
out. And that external portion is

317
00:22:16,000 --> 00:22:21,000
called an antigen. So,
there's a specific chemical

318
00:22:21,000 --> 00:22:25,000
structure which is being
recognized by the antiserum, and we

319
00:22:25,000 --> 00:22:30,000
call that an antigen. In
principal, there are dozens of

320
00:22:30,000 --> 00:22:34,000
distinct antigens on the
surface of a poliovirus particle,

321
00:22:34,000 --> 00:22:39,000
each of which one can, in principal,
make an immunizing antibody against.

322
00:22:39,000 --> 00:22:44,000
Each of those can therefore
be considered to be an antigen.

323
00:22:44,000 --> 00:22:48,000
And, how can we imagine this
antiserum as actually successfully

324
00:22:48,000 --> 00:22:53,000
neutralizing the virus particle?
And here, this depended on actually

325
00:22:53,000 --> 00:22:58,000
the biochemical characterization
of antisera because one soon came to

326
00:22:58,000 --> 00:23:03,000
learn that within antisera
are what we call antibodies.

327
00:23:03,000 --> 00:23:07,000
Or, keep in mind that in biology
you never should ever use a simple

328
00:23:07,000 --> 00:23:11,000
Anglo-Saxon word if you can use a
much more complicated Greek or Latin

329
00:23:11,000 --> 00:23:16,000
one. So, we can always
call these immunoglobulins.

330
00:23:16,000 --> 00:23:20,000
You see the word right there.
Immunoglobulin represents an

331
00:23:20,000 --> 00:23:25,000
antibody molecule, and it
turns out that much of the

332
00:23:25,000 --> 00:23:29,000
protein in the soluble fraction
of our serum consists of such

333
00:23:29,000 --> 00:23:34,000
antibody molecules. And these
antibody molecules have a

334
00:23:34,000 --> 00:23:38,000
very interesting structure which
I'd like to dwell upon momentarily.

335
00:23:38,000 --> 00:23:42,000
But before I do that, I'd just want
to anticipate what I'm about to say

336
00:23:42,000 --> 00:23:46,000
by indicating that we now realize
that an antibody molecule can

337
00:23:46,000 --> 00:23:50,000
recognize an antigen on the
surface of the virus particle,

338
00:23:50,000 --> 00:23:54,000
and the antibody molecule can
actually physically bind to this

339
00:23:54,000 --> 00:23:58,000
antigen. So, we have an
antigen antibody complex,

340
00:23:58,000 --> 00:24:02,000
and that binding of the antibody
molecule to the surface of the virus

341
00:24:02,000 --> 00:24:06,000
particle is what results
functionally in the neutralization

342
00:24:06,000 --> 00:24:11,000
of its infectivity. Or
to put it another way,

343
00:24:11,000 --> 00:24:15,000
given the fact that this particular
capsid protein might be repeated,

344
00:24:15,000 --> 00:24:19,000
let's say, 60 times around a
spherical surface of a virus

345
00:24:19,000 --> 00:24:23,000
particle, there might be, in
fact, 60 different antibody

346
00:24:23,000 --> 00:24:27,000
molecules recognizing this repeated
antigen which occurs on each of

347
00:24:27,000 --> 00:24:31,000
these capsid proteins.
And therefore,

348
00:24:31,000 --> 00:24:35,000
you can imagine in a very
approximate way that if you have an

349
00:24:35,000 --> 00:24:39,000
effective neutralizing antibody,
it's coating the surface of the

350
00:24:39,000 --> 00:24:42,000
virus particle of antibody molecules,
and that clearly obstructs the

351
00:24:42,000 --> 00:24:46,000
ability of the virus particle
to initiate a subsequent round of

352
00:24:46,000 --> 00:24:49,000
infection. Here's what a single
antibody molecule looks like,

353
00:24:49,000 --> 00:24:53,000
and it has some critically
important features. First of all,

354
00:24:53,000 --> 00:24:57,000
there is a specific region of
the antibody that recognizes

355
00:24:57,000 --> 00:25:01,000
the antigen. In this
case, I'm using as the

356
00:25:01,000 --> 00:25:05,000
example the antigen an oligopeptide
on the surface of the poliovirus

357
00:25:05,000 --> 00:25:09,000
particle. And that is located in
this part of the antibody molecule.

358
00:25:09,000 --> 00:25:14,000
Note, by the way, the antibody
molecule is a heterotetramer.

359
00:25:14,000 --> 00:25:18,000
It has two light chains, two
small chains here on the right,

360
00:25:18,000 --> 00:25:23,000
and two heavy chains. Note that
the cysteine disulphide bonds are

361
00:25:23,000 --> 00:25:27,000
holding the entire assembly together.
So here, one is not relying on

362
00:25:27,000 --> 00:25:32,000
hydrogen bonds to hold
together this heterotetramer.

363
00:25:32,000 --> 00:25:36,000
And note the following, that
there are portions of the

364
00:25:36,000 --> 00:25:41,000
antibody molecule which may
be specifically varied from one

365
00:25:41,000 --> 00:25:46,000
antibody molecule to another over
here, and other portions which are

366
00:25:46,000 --> 00:25:50,000
standard operating hardware. And
let me elaborate on that for a

367
00:25:50,000 --> 00:25:55,000
moment. If you look in the serum of
an individual and you try to analyze

368
00:25:55,000 --> 00:26:00,000
the chemical structure of the
antibody molecules in their serum,

369
00:26:00,000 --> 00:26:04,000
you find that for a given type
of antibody molecule like this,

370
00:26:04,000 --> 00:26:09,000
all the antibody molecules
share this constant here: light

371
00:26:09,000 --> 00:26:13,000
purple domain. They
all have that in common.

372
00:26:13,000 --> 00:26:17,000
They all have in common
also the light blue domains.

373
00:26:17,000 --> 00:26:21,000
But if you were able, and
we'll discuss this in a moment

374
00:26:21,000 --> 00:26:25,000
how you could do so, if you
were able to analyze this

375
00:26:25,000 --> 00:26:29,000
other portion, you would
find that when you look at

376
00:26:29,000 --> 00:26:33,000
one antibody molecule, it has
a certain amino acid sequence

377
00:26:33,000 --> 00:26:37,000
here, which is involved in antigen
recognition, and that this amino

378
00:26:37,000 --> 00:26:41,000
acid sequence on the light and heavy
chain varies from one heterotetramer

379
00:26:41,000 --> 00:26:45,000
to another heterotetramer. And
again, I'm going to reinforce

380
00:26:45,000 --> 00:26:50,000
that. But I want to indicate
that there are, within the human

381
00:26:50,000 --> 00:26:55,000
antiserum, millions of structurally
distinct antibody molecules.

382
00:26:55,000 --> 00:27:00,000
They share in common the structure
down here up to this point.

383
00:27:00,000 --> 00:27:03,000
And thereafter, each
of them has its own

384
00:27:03,000 --> 00:27:06,000
idiosyncratic,
its own unusual,

385
00:27:06,000 --> 00:27:09,000
its own particular private
antigen recognition site,

386
00:27:09,000 --> 00:27:13,000
which stems from a region or a
pocket of the protein which is

387
00:27:13,000 --> 00:27:16,000
involved in antigen recognition and
has variable amino acid sequences.

388
00:27:16,000 --> 00:27:19,000
Here, you see the way that an x-ray
crystallographer would actually

389
00:27:19,000 --> 00:27:23,000
visualize this molecule, and
you begin to realize that the

390
00:27:23,000 --> 00:27:26,000
schematic that I showed you before,
in fact, is really doing a little

391
00:27:26,000 --> 00:27:29,000
bit of violence to the
real amino acid sequence,

392
00:27:29,000 --> 00:27:33,000
to the real three dimensional
structure, excuse me.

393
00:27:33,000 --> 00:27:36,000
Now, note something else that is
implicit in what I've just said but

394
00:27:36,000 --> 00:27:40,000
I haven't said it explicitly,
there are actually two antigen

395
00:27:40,000 --> 00:27:44,000
recognition sites,
one here and one here.

396
00:27:44,000 --> 00:27:48,000
So, this antibody is in that
sense bivalent. And if there were,

397
00:27:48,000 --> 00:27:51,000
for example, two poliovirus
particles, one over here and one

398
00:27:51,000 --> 00:27:55,000
over here, you could imagine in
principal, although I'm not drawing

399
00:27:55,000 --> 00:27:59,000
things to scale, that this
antibody molecule could

400
00:27:59,000 --> 00:28:03,000
actually use one of its antigen
recognition sites to bind over here

401
00:28:03,000 --> 00:28:07,000
to one poliovirus particle,
and this to bind to another

402
00:28:07,000 --> 00:28:11,000
poliovirus particle. Now,
I've told you that the antigen

403
00:28:11,000 --> 00:28:15,000
is recognized by this antibody is
an oligopeptide, which constitutes a

404
00:28:15,000 --> 00:28:20,000
distinct chemical structure. How
many different oligopeptides of

405
00:28:20,000 --> 00:28:25,000
ten amino acids long are there
in the mathematical universe?

406
00:28:25,000 --> 00:28:30,000
How many conceivable
ones are there?

407
00:28:30,000 --> 00:28:34,000
How many amino acids are there?
20? So what's the number? I heard

408
00:28:34,000 --> 00:28:38,000
you all say 1020, right?
So, that's an awful lot.

409
00:28:38,000 --> 00:28:42,000
That's more than you can shake a
stick at. So that means that there

410
00:28:42,000 --> 00:28:46,000
are, in principal, essentially
an infinite number of

411
00:28:46,000 --> 00:28:50,000
oligopeptides of ten amino acids
long, and each one of those could,

412
00:28:50,000 --> 00:28:54,000
in principal constitute a distinct
antigen that could be recognized by

413
00:28:54,000 --> 00:28:58,000
the antibody molecule. Note
that what's happening here is

414
00:28:58,000 --> 00:29:02,000
that a protein antigen such as
one of these oligopeptides is being

415
00:29:02,000 --> 00:29:06,000
recognized by another protein,
which is the antibody molecule.

416
00:29:06,000 --> 00:29:10,000
And therefore, we begin
to imagine there's some

417
00:29:10,000 --> 00:29:14,000
kind of lock and key complementarity
where somehow the amino acid

418
00:29:14,000 --> 00:29:18,000
sequence here recognizes and binds a
complementary fashion to the antigen

419
00:29:18,000 --> 00:29:22,000
that is being recognized, and
there's a physical association

420
00:29:22,000 --> 00:29:26,000
which allows the antibody molecule
then to attach tightly to the

421
00:29:26,000 --> 00:29:30,000
antigen bearing
poliovirus particle.

422
00:29:30,000 --> 00:29:34,000
And, by the way, to
put it another way,

423
00:29:34,000 --> 00:29:39,000
it could be that some of these
poliovirus capsid proteins have not

424
00:29:39,000 --> 00:29:43,000
yet been assembled in a virus
particle. And even if they aren't,

425
00:29:43,000 --> 00:29:48,000
in principal a free protein,
a capsid protein of poliovirus,

426
00:29:48,000 --> 00:29:53,000
could also be recognized by an
antibody molecule and be bound by

427
00:29:53,000 --> 00:29:57,000
the antibody molecule. Now,
this creates several major

428
00:29:57,000 --> 00:30:02,000
conceptual problems. How
on earth can the body make

429
00:30:02,000 --> 00:30:06,000
antibody molecules that are able
to recognize a poliovirus particle?

430
00:30:06,000 --> 00:30:10,000
Well, you'll say that's easy.
Clearly, in our genes there must be

431
00:30:10,000 --> 00:30:14,000
some kind of nucleotide sequence
which has up here the ability encode

432
00:30:14,000 --> 00:30:19,000
an antigen-binding site.
Note, by the way, the

433
00:30:19,000 --> 00:30:23,000
antigen-binding site's actually
composed of two different proteins.

434
00:30:23,000 --> 00:30:27,000
Here's part of the antigen-binding
site. Here's another part of the

435
00:30:27,000 --> 00:30:31,000
antigen-binding site. So,
it must be on two different

436
00:30:31,000 --> 00:30:35,000
genes. But, these two different
genes create an antigen-binding site

437
00:30:35,000 --> 00:30:39,000
which can recognize poliovirus.
Bind the poliovirus antigen and

438
00:30:39,000 --> 00:30:43,000
neutralize it. Well,
that's well and good.

439
00:30:43,000 --> 00:30:47,000
That's perfectly reasonable,
but let me tell you what's

440
00:30:47,000 --> 00:30:51,000
unreasonable about that.
Each of us during his or her

441
00:30:51,000 --> 00:30:55,000
lifetime are going to be infected
by literally hundreds of viruses.

442
00:30:55,000 --> 00:30:59,000
You get cold viruses almost
every two, three, four, five

443
00:30:59,000 --> 00:31:03,000
times a year. By
the time you get old,

444
00:31:03,000 --> 00:31:07,000
you actually have acquired
immunity to many of these,

445
00:31:07,000 --> 00:31:11,000
but during the course of one's
lifetime, the immune system has been

446
00:31:11,000 --> 00:31:15,000
able to generate antiviral immunity
and eliminate virtually all the

447
00:31:15,000 --> 00:31:19,000
viral infections you ever got.
How many people do you know who

448
00:31:19,000 --> 00:31:23,000
have actually died of a viral
infection? And yet each one of

449
00:31:23,000 --> 00:31:27,000
these viruses is in principal able
to replicate throughout your body

450
00:31:27,000 --> 00:31:32,000
and kill you, and they never do?
So that means that the immune system

451
00:31:32,000 --> 00:31:36,000
is extremely effective in developing
an antibody or antibodies which

452
00:31:36,000 --> 00:31:40,000
could neutralize infecting virus
particles. You might have the cold

453
00:31:40,000 --> 00:31:44,000
for a day, a week, or two
weeks, but eventually the

454
00:31:44,000 --> 00:31:48,000
immune system develops enough
antibodies and other strategies to

455
00:31:48,000 --> 00:31:52,000
eliminate the viral infections
from your body. I remember when my

456
00:31:52,000 --> 00:31:56,000
grandfather was 94 years old and
he got a cold. And he hadn't had a

457
00:31:56,000 --> 00:32:00,000
cold for 20 years because by the age
of 75, he had been exposed to almost

458
00:32:00,000 --> 00:32:04,000
every virus imaginable. And
so, when he got a cold at the

459
00:32:04,000 --> 00:32:08,000
age of 94, his brain was still
working, he'd forgotten how to blow

460
00:32:08,000 --> 00:32:12,000
his nose. It just hadn't happened
to him. He didn't know what it was

461
00:32:12,000 --> 00:32:16,000
like to have a cold anymore.
So, it just goes to show you that

462
00:32:16,000 --> 00:32:20,000
the immune system can be very
versatile and very successful.

463
00:32:20,000 --> 00:32:24,000
But this creates a major conceptual
problem. How do our genes know how

464
00:32:24,000 --> 00:32:28,000
to create antigen-binding sites in
our antibodies that recognize all of

465
00:32:28,000 --> 00:32:32,000
the infectious viruses that
are going to attack us during

466
00:32:32,000 --> 00:32:36,000
a lifetime. Do we have
a gene for neutralizing

467
00:32:36,000 --> 00:32:40,000
smallpox? Do we have a gene
for neutralizing poliovirus and

468
00:32:40,000 --> 00:32:44,000
adenovirus, which gives us bad
head colds. And, rabies virus and

469
00:32:44,000 --> 00:32:48,000
vesicular stomatitis virus, and
all other kinds of viruses that

470
00:32:48,000 --> 00:32:52,000
we are going to experience
in a lifetime. Well,

471
00:32:52,000 --> 00:32:56,000
possibly we do have a gene
for each one of those. But,

472
00:32:56,000 --> 00:33:00,000
then I'm beginning to raise some
other questions if you believe that,

473
00:33:00,000 --> 00:33:04,000
which you shouldn't. First
of all, there are many

474
00:33:04,000 --> 00:33:08,000
different distinct oligopeptide
antigens on the surface of the

475
00:33:08,000 --> 00:33:12,000
poliovirus particle. If you
look at the antibodies that

476
00:33:12,000 --> 00:33:16,000
had been developed against a
poliovirus capsid protein in an

477
00:33:16,000 --> 00:33:19,000
immunized individual,
they have one antibody that

478
00:33:19,000 --> 00:33:23,000
recognizes one part of these capsid
protein, and another antibody that

479
00:33:23,000 --> 00:33:27,000
recognized another part.
So, here's the surface of the

480
00:33:27,000 --> 00:33:31,000
capsid protein blown up now,
and we can imagine there are

481
00:33:31,000 --> 00:33:35,000
different oligopeptide epitopes
on different parts of the protein.

482
00:33:35,000 --> 00:33:39,000
And they're all ten amino acids
long, for example. The fact is,

483
00:33:39,000 --> 00:33:43,000
in individuals who have
anti-poliovirus antibody,

484
00:33:43,000 --> 00:33:48,000
you can find that there's an
antibody that recognizes this

485
00:33:48,000 --> 00:33:52,000
antigen, and one that recognizes
this antigen, and another that

486
00:33:52,000 --> 00:33:57,000
recognizes this antigen. So,
the number of distinct antigens

487
00:33:57,000 --> 00:34:01,000
which the immune system has
succeeded in making antibodies

488
00:34:01,000 --> 00:34:05,000
against, vastly exceeds the number
of infectious agents we're going to

489
00:34:05,000 --> 00:34:10,000
experience in a lifetime. And,
that begins to undermine your

490
00:34:10,000 --> 00:34:14,000
confidence in the notion that when
we're born, we already possess the

491
00:34:14,000 --> 00:34:18,000
genes to recognize
different infectious agents,

492
00:34:18,000 --> 00:34:22,000
and to construct neutralizing
antibodies against them.

493
00:34:22,000 --> 00:34:26,000
There's also another flaw in this
whole notion that we inherit a set

494
00:34:26,000 --> 00:34:30,000
of genes that enable us from
the get-go to make specific

495
00:34:30,000 --> 00:34:34,000
antibodies. And that
flaw comes from the

496
00:34:34,000 --> 00:34:38,000
following notion. Evolution
cannot anticipate which

497
00:34:38,000 --> 00:34:42,000
infectious agents each one of us is
going to experience in a lifetime.

498
00:34:42,000 --> 00:34:47,000
Let's say that some of us
experience a totally new kind of a

499
00:34:47,000 --> 00:34:51,000
viral infection which had never
been experienced by our ancestors.

500
00:34:51,000 --> 00:34:55,000
If the way of neutralizing that
virus, and defending us against that

501
00:34:55,000 --> 00:35:00,000
infection depended on using an
antibody molecule whose sequence was

502
00:35:00,000 --> 00:35:04,000
encoded in our germ line in
the genes we inherited at birth,

503
00:35:04,000 --> 00:35:08,000
then we might be in bad shape if
that virus was a novel virus to

504
00:35:08,000 --> 00:35:13,000
which the human race
had never been exposed.

505
00:35:13,000 --> 00:35:17,000
And in arguing that, I'm
telling you that it's impossible

506
00:35:17,000 --> 00:35:21,000
to imagine a situation where our
germ line, the set of genes we are

507
00:35:21,000 --> 00:35:26,000
born with that we start life with,
already contains the information for

508
00:35:26,000 --> 00:35:30,000
making each one of the different
kinds of antibodies that

509
00:35:30,000 --> 00:35:34,000
are in our antiserum. How
many different kinds of

510
00:35:34,000 --> 00:35:38,000
antibodies are in our antiserum?
Well now, the whole notion becomes

511
00:35:38,000 --> 00:35:42,000
even more stark because
there's probably millions,

512
00:35:42,000 --> 00:35:46,000
maybe even ten or 100 million
distinct antibody molecules floating

513
00:35:46,000 --> 00:35:50,000
around in our blood. Each
one in this case has the same

514
00:35:50,000 --> 00:35:54,000
constant region down here.
You see the constant region,

515
00:35:54,000 --> 00:35:58,000
and each one of these 100 million
distinct antibody species has its

516
00:35:58,000 --> 00:36:01,000
own antigen-combining site.
Now, if we want to pursue that

517
00:36:01,000 --> 00:36:05,000
further, we have to begin to imagine
how the immune system can make so

518
00:36:05,000 --> 00:36:09,000
many different antibody molecules.
This itself is a major conceptual

519
00:36:09,000 --> 00:36:13,000
challenge. And now we have to
go and know a little bit of cell

520
00:36:13,000 --> 00:36:16,000
biology because if we
look at the immune system,

521
00:36:16,000 --> 00:36:20,000
and there are cells forming
in the immune system,

522
00:36:20,000 --> 00:36:24,000
we find a set of cells that are
called B cells or a subset of B

523
00:36:24,000 --> 00:36:28,000
cells that are called plasma cells.
And the plasma cells, and there's a

524
00:36:28,000 --> 00:36:32,000
whole bunch of
different plasma cells.

525
00:36:32,000 --> 00:36:35,000
So the plasma cells are floating
around in the circulation,

526
00:36:35,000 --> 00:36:39,000
and the plasma cells are able to
secrete antibody molecules into the

527
00:36:39,000 --> 00:36:43,000
serum. And they are abundant.
They're around by the billions

528
00:36:43,000 --> 00:36:47,000
inside our body. And
these plasma cells are

529
00:36:47,000 --> 00:36:51,000
constantly secreting antibody
molecules into the serum around them

530
00:36:51,000 --> 00:36:55,000
into the plasma around them.
And these are the antibody

531
00:36:55,000 --> 00:36:59,000
molecules I've been talking about
here, the antibody molecules that

532
00:36:59,000 --> 00:37:03,000
are capable of neutralizing
poliovirus particles.

533
00:37:03,000 --> 00:37:06,000
So, this now creates
another conceptual question.

534
00:37:06,000 --> 00:37:10,000
Let's imagine for the sake of
argument that in our blood stream

535
00:37:10,000 --> 00:37:13,000
there are 100 million distinct
antigen species floating around.

536
00:37:13,000 --> 00:37:17,000
And I don't mean 100 million
molecules. I mean there's a million

537
00:37:17,000 --> 00:37:21,000
molecules, and each one of
these species has a different

538
00:37:21,000 --> 00:37:24,000
antigen-binding site.
So, there may be many

539
00:37:24,000 --> 00:37:28,000
antigen-binding sites.
There may be many antibody

540
00:37:28,000 --> 00:37:32,000
molecules. There may be
million of this kind of

541
00:37:32,000 --> 00:37:36,000
antibody that recognize this
antigen. There may be millions of this

542
00:37:36,000 --> 00:37:40,000
antibody recognizing this
antigen, millions of this antibody

543
00:37:40,000 --> 00:37:44,000
recognizing this antigen on
the surface of poliovirus.

544
00:37:44,000 --> 00:37:48,000
And I'm saying there could be
a million distinct species of

545
00:37:48,000 --> 00:37:52,000
antibodies, each species being
defined by the antigen that

546
00:37:52,000 --> 00:37:56,000
recognizes and by the structure
of its antigen-recognizing site,

547
00:37:56,000 --> 00:38:00,000
its antigen-binding
site. In other words,

548
00:38:00,000 --> 00:38:03,000
to repeat myself, what
distinguishes one species of

549
00:38:03,000 --> 00:38:06,000
antibodies from another
is the amino acid sequence,

550
00:38:06,000 --> 00:38:09,000
the structure of this particular
region of the antibody molecule that

551
00:38:09,000 --> 00:38:12,000
makes one species of antibody
different from the other.

552
00:38:12,000 --> 00:38:16,000
And with all that in mind, we
have two alternative scenarios.

553
00:38:16,000 --> 00:38:19,000
Let's again say there's a million
different species of antibodies

554
00:38:19,000 --> 00:38:22,000
being secreted into the antiserum
at any one point in time.

555
00:38:22,000 --> 00:38:25,000
This cell could make all million.
This cell could make the whole

556
00:38:25,000 --> 00:38:28,000
million species.
This cell could,

557
00:38:28,000 --> 00:38:32,000
and this cell could.
In other words,

558
00:38:32,000 --> 00:38:38,000
each one of these B cells
could be multitalented,

559
00:38:38,000 --> 00:38:43,000
able to simultaneously make a
million different kinds of antibody

560
00:38:43,000 --> 00:38:49,000
molecules. The opposite model is
as follows, and that is that this B

561
00:38:49,000 --> 00:38:54,000
cell makes one species of antibody.
We'll call it antibody A, and this

562
00:38:54,000 --> 00:39:00,000
species makes another
species of antibody molecule.

563
00:39:00,000 --> 00:39:04,000
So, this B cell makes antibody A.
This B cell makes antibody B. This

564
00:39:04,000 --> 00:39:08,000
B cell makes antibody C. And
keep in mind that when I'm

565
00:39:08,000 --> 00:39:12,000
saying antibody A, B, and
C, I mean A has a certain

566
00:39:12,000 --> 00:39:17,000
antigen-binding site. Be has
another antigen binding site.

567
00:39:17,000 --> 00:39:21,000
C has yet another antigen binding
site. And, we can distinguish

568
00:39:21,000 --> 00:39:25,000
between these two alternative
mechanistic models by looking at the

569
00:39:25,000 --> 00:39:29,000
disease called multiple myeloma.
And what you have in multiple

570
00:39:29,000 --> 00:39:33,000
myeloma is the following.
All of a sudden,

571
00:39:33,000 --> 00:39:37,000
the blood stream becomes full of
much greatly elevated levels of

572
00:39:37,000 --> 00:39:41,000
antibody molecules. They're
present in the blood stream.

573
00:39:41,000 --> 00:39:45,000
Now, normally, if you
look at all the antibody

574
00:39:45,000 --> 00:39:48,000
molecules in the blood stream and
you were to separate them by some

575
00:39:48,000 --> 00:39:52,000
kind of electrophoretic technique
which distinguished them on the

576
00:39:52,000 --> 00:39:56,000
basis of subtle differences
in the amino acid sequence,

577
00:39:56,000 --> 00:40:00,000
and let's not worry about
exactly how you do that.

578
00:40:00,000 --> 00:40:03,000
But we've talked about a million
different antibody species.

579
00:40:03,000 --> 00:40:07,000
They're chemically slightly
different from one another by virtue

580
00:40:07,000 --> 00:40:11,000
of the antigen-recognizing site.
And if you could separate them by

581
00:40:11,000 --> 00:40:15,000
some kind of system that separated
them on the basis of their charge,

582
00:40:15,000 --> 00:40:18,000
you'd find a whole spectrum of
different antibody molecules,

583
00:40:18,000 --> 00:40:22,000
a million in this large,
heterogeneous mixture of antibody

584
00:40:22,000 --> 00:40:26,000
molecules, each species found
somewhere or another in this very

585
00:40:26,000 --> 00:40:30,000
broad distribution of 100 million
distinct species which are all mixed

586
00:40:30,000 --> 00:40:33,000
together in the antiserum. If
you look at the serum or the

587
00:40:33,000 --> 00:40:37,000
plasma of a patient suffering
from multiple myeloma,

588
00:40:37,000 --> 00:40:41,000
what you find is the following,
that one species of antibody

589
00:40:41,000 --> 00:40:45,000
molecules dominates over all
the other ones. And so now,

590
00:40:45,000 --> 00:40:49,000
whereas that single species might
only be on average one millionth of

591
00:40:49,000 --> 00:40:53,000
the total antibody complement in
the blood. In those suffering from

592
00:40:53,000 --> 00:40:57,000
myeloma, or sometimes it's
called multiple myeloma,

593
00:40:57,000 --> 00:41:01,000
now one single antibody
species may represent 60, 70,

594
00:41:01,000 --> 00:41:05,000
or 80% of all the antibody
molecules in the blood.

595
00:41:05,000 --> 00:41:09,000
And clearly something has gone
very wrong. And if you look at what

596
00:41:09,000 --> 00:41:14,000
happened in the case of an
individual of multiple myeloma,

597
00:41:14,000 --> 00:41:18,000
if we look at that diagram up there,
what you see now is that instead of

598
00:41:18,000 --> 00:41:23,000
there being many different B cells
that are equivalently represented in

599
00:41:23,000 --> 00:41:28,000
the blood stream, now one
of the B cell types has

600
00:41:28,000 --> 00:41:32,000
begun to multiple uncontrollably,
and this vasicular B cell type,

601
00:41:32,000 --> 00:41:37,000
which I indicate up there,
which happens to be specifically

602
00:41:37,000 --> 00:41:42,000
able to make antibody C in this
model, now this particular species

603
00:41:42,000 --> 00:41:46,000
of B cells is now a predominant
constituent of the population of B

604
00:41:46,000 --> 00:41:50,000
cells in the blood.
So just to reiterate,

605
00:41:50,000 --> 00:41:54,000
in the normal blood, there are a
million different sub-populations of

606
00:41:54,000 --> 00:41:58,000
B cells. But in these individuals,
one of the sub-populations of B

607
00:41:58,000 --> 00:42:01,000
cells has expanded. It's
created a monoclonal growth.

608
00:42:01,000 --> 00:42:05,000
Keep in mind monoclonal refers to
the fact that all the cells in this

609
00:42:05,000 --> 00:42:09,000
tumor, and there is a tumor,
descend from the same ancestor.

610
00:42:09,000 --> 00:42:13,000
So, it's a monoclonal
growth. It's a kind of cancer.

611
00:42:13,000 --> 00:42:17,000
It's a kind of blood cancer.
And all the cells in this

612
00:42:17,000 --> 00:42:21,000
population make the
identical antibody molecule,

613
00:42:21,000 --> 00:42:25,000
which explains this very homogeneous
peak here riding above the very

614
00:42:25,000 --> 00:42:29,000
heterogeneous mixture of antibodies,
the background heterogeneity that

615
00:42:29,000 --> 00:42:33,000
normally characterizes
normal serum.

616
00:42:33,000 --> 00:42:36,000
And what that clearly indicates is
that each different B cell or plasma

617
00:42:36,000 --> 00:42:40,000
cell as you will like to call it in
our normal serum is specialized to

618
00:42:40,000 --> 00:42:44,000
make its own particular kind of
antibody. It's not that a single B

619
00:42:44,000 --> 00:42:48,000
cell can make a million different
antibodies. Each B cell makes a

620
00:42:48,000 --> 00:42:52,000
very specific kind of antibody.
And when I say a specific antibody

621
00:42:52,000 --> 00:42:56,000
or a specific antibody species,
again I'm referring to an antibody

622
00:42:56,000 --> 00:43:00,000
that has a specific
antigen-binding site.

623
00:43:00,000 --> 00:43:04,000
Or to put it another way, all
of the antibody molecules coming

624
00:43:04,000 --> 00:43:09,000
out of a single B cell are
identical with one another.

625
00:43:09,000 --> 00:43:14,000
That, then, raises the question of
how the B cell learns how to make

626
00:43:14,000 --> 00:43:19,000
that antibody molecule and
not other antibody molecules.

627
00:43:19,000 --> 00:43:23,000
There's yet another puzzle we have
to answer, and that's the following.

628
00:43:23,000 --> 00:43:28,000
If you look at the serum response
of an individual to a viral

629
00:43:28,000 --> 00:43:32,000
infection, it looks like this.
And here, let's look at what's

630
00:43:32,000 --> 00:43:36,000
present in the ordinate, and
it's on a log scale as you can

631
00:43:36,000 --> 00:43:40,000
see here, and what's present on
the abscissa. If one is initially

632
00:43:40,000 --> 00:43:44,000
exposed to an antigen and develops
an immunity, then here is the wave

633
00:43:44,000 --> 00:43:48,000
of antibody production and the
concentration of antibody against,

634
00:43:48,000 --> 00:43:52,000
let's say, poliovirus that is
present in the blood of a recently

635
00:43:52,000 --> 00:43:56,000
infected individual. They're
exposed to the antigen,

636
00:43:56,000 --> 00:44:00,000
or in this case the
poliovirus capsid protein.

637
00:44:00,000 --> 00:44:04,000
In the days and weeks that follow,
they develop a significant level of

638
00:44:04,000 --> 00:44:09,000
antibody which is used to neutralize,
in this case the infected poliovirus

639
00:44:09,000 --> 00:44:13,000
particles. And then once the
poliovirus is cleared from the

640
00:44:13,000 --> 00:44:18,000
system, i.e. once all the infectious
particles have been neutralized,

641
00:44:18,000 --> 00:44:23,000
then the antibody molecules
go down almost to zero,

642
00:44:23,000 --> 00:44:28,000
and there's virtually no antibody
against poliovirus left around.

643
00:44:28,000 --> 00:44:32,000
But look what happens when
that individual is re-exposed to

644
00:44:32,000 --> 00:44:37,000
poliovirus years later. All
of a sudden, now he or she

645
00:44:37,000 --> 00:44:41,000
mounts a massive antibody response
vastly higher than the initial one.

646
00:44:41,000 --> 00:44:46,000
Well, you say it's only
twofold higher than this.

647
00:44:46,000 --> 00:44:50,000
But let's look at the ordinate.
This is a logarithmic plot. So,

648
00:44:50,000 --> 00:44:55,000
on this plot, the secondary antibody
response is maybe 100 or 200 times

649
00:44:55,000 --> 00:45:00,000
more vigorous, much
higher antibody production.

650
00:45:00,000 --> 00:45:04,000
Let's think about this time years
later when that person is exposed to

651
00:45:04,000 --> 00:45:08,000
a second virus. Let's
say that person is exposed

652
00:45:08,000 --> 00:45:13,000
subsequently to cold virus,
adenovirus. Does the exposure to

653
00:45:13,000 --> 00:45:17,000
poliovirus here in the blue
curve help that person develop an

654
00:45:17,000 --> 00:45:21,000
adenovirus antibody response?
The answer is absolutely not.

655
00:45:21,000 --> 00:45:26,000
There's no cross-immunity. And so,
when that individual years later

656
00:45:26,000 --> 00:45:30,000
becomes exposed to adenovirus,
there's once again the same kind of

657
00:45:30,000 --> 00:45:34,000
initial response that happened as
a consequence of previously being

658
00:45:34,000 --> 00:45:39,000
exposed to poliovirus.
It's a rather weak response.

659
00:45:39,000 --> 00:45:43,000
It's effective enough in developing
the initial virus eliminating

660
00:45:43,000 --> 00:45:47,000
ability, but it's not very strong.
What is this telling us? I'm glad

661
00:45:47,000 --> 00:45:51,000
I asked that question. It's
telling us that that immune

662
00:45:51,000 --> 00:45:55,000
system is able to remember
over a period of months,

663
00:45:55,000 --> 00:45:59,000
years, and decades that a previous
exposure to this antigen has

664
00:45:59,000 --> 00:46:03,000
occurred because here the immune
system in this second red curve

665
00:46:03,000 --> 00:46:07,000
knows, somehow remembers, that
there was an exposure years

666
00:46:07,000 --> 00:46:12,000
earlier. And not only
does it remember and

667
00:46:12,000 --> 00:46:16,000
respond rapidly, but it
responds much more vigorously.

668
00:46:16,000 --> 00:46:20,000
And here now, we begin
to recognize some of the

669
00:46:20,000 --> 00:46:24,000
outlines of immunity because the
immune system remembers this earlier

670
00:46:24,000 --> 00:46:28,000
exposure, and when it's provoked a
second time, through an accidental

671
00:46:28,000 --> 00:46:32,000
and inadvertent exposure to
poliovirus on a second occasion,

672
00:46:32,000 --> 00:46:36,000
now it really goes to town. And
now it creates antisera which is

673
00:46:36,000 --> 00:46:40,000
much more vigorous than it
was during the first exposure.

674
00:46:40,000 --> 00:46:44,000
If the first exposure had never
occurred, there wouldn't be such a

675
00:46:44,000 --> 00:46:47,000
vigorous response. Look
at this one over here.

676
00:46:47,000 --> 00:46:51,000
So now, we begin to realize two
mysteries that we have to explain

677
00:46:51,000 --> 00:46:55,000
next time in our further
discussion of the immune system.

678
00:46:55,000 --> 00:46:59,000
First of all, how do B cells figure
out how to make so many different

679
00:46:59,000 --> 00:47:03,000
kinds of antibody molecules.
And secondly, how does the immune

680
00:47:03,000 --> 00:47:07,000
system remember from one decade
to the next, that exposure to a

681
00:47:07,000 --> 00:47:12,000
previous antigen has occurred
which merits a vigorous response.

682
00:47:12,000 --> 00:47:17,000
See you then
on Friday.