In this video, I'd like to give an overview
of how the B and T cells in your body
are prepared to fight infections.
And so, let's start with where B cells come from.
So where do B cells come from?
They come from the bone marrow.
So, here's a bone and inside it we have some
bone marrow, and it's from there that B cells come from.
But the process by which the B cells are
prepared in the bone marrow is actually
very interesting and unique in terms of the human body.
Because well, like many other places in the body,
the B cells divide and form millions of
descendants and that is how your whole body
grows and comes to be as big as it is.
But, for B cells there's something very
different from other parts of your body,
and that is that each daughter cell that
grows up in the bone marrow is different from
its parent and is also different from all of its siblings.
So, just to remind you, B cells have these
receptors that later on will be their antibodies.
But each daughter cell of this original
B cell will have a different receptor.
And that's because, in the process of being made,
these daughter B cells will have a slight
shuffling, a slight changing in their DNA,
but specifically for this receptor, and so
by the time they're created, they have a
unique receptor that will identify a unique
criminal, a unique bacterium or
virus or something in the body.
That receptor will be useless for all
the other criminals in the body.
So, it's very specific but very powerful
if it finds the thing that it wants to kill.
So, this is happening in the bone marrow.
And, basically the same exact thing is
happening for T cells, except it's not
happening in the bone marrow, that's happening here.
You know what this is called?
This is called the thymus and I'm
drawing it a little bit bigger than I should,
maybe, but that way you can see it clearly.
The thymus has two lobes or two parts,
and it's located right behind your sternum.
So, T cells come from the thymus.
And, very similar to B cells, they have a unique receptor.
It's not quite the same and it
doesn't become an antibody later on.
They have a unique receptor and, while
these T cells are dividing and creating
millions of daughter T cells, each of
these daughter T cells will have its own
unique receptor that will also identify a
unique criminal or bacterium in the human body.
And we remember that these receptors
actually find antigens, so pieces of these
criminals that are presented to them by
antigen presenting cells and we'll revisit that in a second.
So, all this is occurring in the thymus.
And so, this is step one of the process and
it's unique to the human body because we
end up with all these actually, literally
genetically different B and T cells that are
very specific to things which may or may not even
exist out there because they're really created at random.
Now, the next thing that happens is that
these B cells and T cells, once they're ready,
are going to migrate to a lymph node, and
you have lymph nodes all over your body, about 600 of them.
But, let's say that they go to one here, in your armpit.
They're going to migrate to that lymph node,
for example, and that is where they're
going to wait for their special
criminal that they can react to.
So, they're going to go over there to that lymph node.
And let's draw that down here, maybe significantly bigger.
So here's the lymph node, and you're going
to have your B and T cells, all the
different kinds of them, lying in wait there.
And they have their special receptors.
Each one is specific to something else.
So, these guys have migrated here to all
the different lymph nodes of the body.
And now these guys are ready, and so the
next step is really infection, so let's
look at a tissue in your body with some cells.
These are cells, let's say it's over there.
So it's not too far from that lymph node there.
And let's say it gets infected.
So here are some bacteria which are causing damage.
And one of the things we have out here
is these cells, which are part of the immune system,
which are called dendritic cells, dendritic cells.
And these cells can eat some of these
bacteria, so they can actually kill some of
those bacteria, they can ingest them.
But, it's actually not quite to the point
that they can defeat this infection and
that's not really their purpose.
And their purpose is actually more to present
these bacteria to the B and T cells
that we've been talking about.
And so, the way that they do that is that
they come here into these lymphatic vessels,
which start here in the tissue, and
they go up to the lymph node where they're
going to present these bacteria to these cells.
And at the same time, some of the bacteria might
also get swept in there and might go into the lymph node.
And so, you get some of them out here.
You get some bacteria and maybe just some
pieces of bacteria that got broken down.
And you also have your dendritic cells here,
which are antigen presenting cells, which
means that they have this protein that
we've talked about called MHC, MHC, on which
they present pieces of those bacteria that they've ingested.
Now these T cells here are unable to identify
these bacteria when they're just freely floating.
They need this antigen presenting cell
to show this piece of bacteria to them.
And only the T cells, which can recognise
this very specific pathogen, are going to
see it and bind to it and react to it.
So in this case, it's this guy.
He's going to bind to that little piece and
recognize that his invader, the one guy
he's been created to kill is somewhere in the body.
Meanwhile, these B cells are made to
react directly to these bacteria
without this antigen presenting cell.
And so, the specific one that is specific
to this bacterium is going to see it,
bind to it and also be alerted.
However, all these other guys?
They're not going to know that anything's going on.
Because, they can't bind to any of these
pathogens that were brought into the lymph node.
And so, this right here is step two.
When the infection occurs, and the
right B and T cells are alerted.
And what are they going to do now?
Well now, they're going to activate.
They're going to jump into action
because finally they're needed.
Now these B and T cells are going
to start replicating like crazy.
So here are the two lucky winners who got selected.
And they're going to say, "well,
"now that we know we are useful for fighting
"this infection, we better replicate like crazy
"because right now there's only two of us
"and that's not nearly enough."
So, they're going to start replicating,
and each of their descendants,
unlike this kind of replication,
now each of their descendants is
going to have the same receptor.
It's going to have the same receptor.
If it had a different receptor, this whole
process would be useless because it wouldn't be able to
fight the specific infection that we've just identified.
And I should mention that not all of
these guys are going to do the same thing.
So, most of them will become effector cells,
which means that they're going to be ready
to fight this infection right now.
But some of them are going to become
memory cells, which means that they're not
going to actually fight right now.
They're going to wait until the next time
that this pathogen comes around.
And the reason why we create these guys is
because if we get this bacteria once,
there's a high chance that we're going to get it again.
Because, really, you create billions and
billions of these different receptors, but
only a very small subset of them will
actually end up being pathogens that you see.
And so, this is step 3 of our process,
which has finally resulted in an army of
B and T cells, ready to fight this specific invader.
So that's what they're going to do now.
The B cells are going to pump out antibodies
to attack this bacterium and the T cells,
some of them are going to stay in the lymph node
and some of them are going to travel out
to this site to join the battle.
And so, this whole process can actually
be called clonal selection, clonal selection.
Which is a name for the theory of this
process when it first came out.
So this is clonal selection and the reason
it's called clonal selection is because
you're going to select B and T cells that
you need and then you're going to start
cloning them like crazy into this army.
And the process involves all these very
unique, specific B and T cells coming from
the bone marrow and thymus to the lymph node,
where they're going to be met by these
bacteria that are infecting your skin,
that are going to travel through the
lymphatic vessels to the lymph node.
And so, they'll meet there and
then the armies will be prepared.