In the very first immunology video, where we talked about
nonspecific defense mechanisms, we said if we had
some type of a pathogen-- let's say it's a bacteria--
that our phagocytes can recognize either proteins on
the bacteria or maybe it was some other type of pathogen--
some other marker on the pathogen.
It doesn't know what type of pathogen it is, but that's
enough for the phagocyte.
It'll engulf the pathogen.
So let's say this is the phagocyte.
Somehow one of their receptors touches one of the proteins
that's on the surface of this bacteria in this case and
says, gee, that's not a good thing to have around and so
it'll engulf it.
So the membrane is just going to surround the pathogen-- and
this part is going to go in.
It's going to kind of pinch in and the pathogen is going to
get surrounded and get engulfed--
phagocytozed by the phagocyte.
So the end product-- we saw this on the first video where
I talked about phagocytes-- is we're going to end up-- so
then the membrane's going to be completely around.
This is going to pinch in and bubble around this thing and
so we're going to have the pathogen sitting
inside of the phagocyte.
It's going to be surrounded by a membrane.
That membrane's called a phagosome.
And we learned there's different types of
phagocytes-- could be a macrophage, could be a
neutrophil, could be a dendritic cell-- and we have
it like this and then we also saw in that video, it's not
just done-- the macrophage doesn't
just digest this thing.
That by itself is very useful.
It got this bacteria out of the way.
If this was a virus, it got that virus out of the way.
But it does even more than that.
It takes that and then it lyses it-- or it breaks it up.
It doesn't have to actually use-- well, it breaks it up.
There's several ways it can do it, but the bottom line is, it
processes it.
It'll break up the pathogen-- and we saw that first video on
phagocytes that we'll actually have a lysosome bond to it and
dump all sorts of particles that are going to break up and
cut up this bacteria, in this case, into
its constituent molecules.
And then some subset of the leftovers, particular chains
of the peptides-- and remember, proteins are long
chains of amino acids.
Polypeptides are short chains.
So you take short chains of those and they're going to
bond to special proteins.
And this is essentially the topic of this video.
So let's say they bond to that special protein, bond to that
special protein.
And then those proteins get transported or get to the
membrane or the outer surface of the cell and they present
themselves along with the piece of the pathogen.
So the end product after phagocytozed this pathogen is
that the phagocyte will look something like this and it
will have these antigen presenting proteins, I guess
we can call them, that had bound to parts of that
original pathogen.
Let me do it right here.
So it has a little bit of the original pathogen on it that I
drew in green right here.
And these proteins right here, these are called-- it's a
fancy word and I talked about it in the previous video--
these are called major histocompatibility complexes,
or MHC for short.
And when we're talking about phagocytes or macrophages or
dendritic cells that are particular cases of
phagocytes, the major histocompatibility complexes
that they present after they've digested this
molecule-- this is an MHC class II.
Let me write this down.
This is an MHC class II protein.
It might seem like I'm really going into the minutiae of
what these proteins are, but we're going to see this is key
for activating other parts of the immune system, especially
the cell mediated parts of the immune system.
So this was the case with a macrophage or a dendrite.
They engulf something, they chew it all up, and then parts
of the chewed up thing that they ate, they attach to these
MHC II proteins and these MHC proteins go to the
surface of the cell.
The same thing-- or actually not the quite the same thing--
a very similar thing happens with B cells.
So if we have a B cell-- that's a good color.
B for blue.
We know B stands for bursa, but it could stand for bone
marrow just as well.
Let's say we have a B cell and it's got its membrane bound
antibody on it.
Remember, it's very specific to that B cells.
So all of the membrane bound antibodies, all 10,000 or so
of them on this B cell, they all expressed the same
variable part.
So this is a particular B cell.
So remember, this was nonspecific.
When we talked about phagocytosis, these guys just
say, you're a bacteria.
You're a virus.
I don't know what kind you are.
I'm just going to eat you up.
You look shady.
I'm going to eat you up.
I don't know what type you are or whether
I've seen you before.
When we're talking about B cells, we're talking about the
adaptive or the specific immune system.
And so these-- the variable ends of these membrane bound
antibodies are specific to certain parts of certain
pathogens, to certain epitopes.
Remember, epitopes were the parts of certain pathogens
that these specific chains can recognize and bond to.
So let's say that we're dealing with a
virus in this situation.
And let's say the virus just happens to
bond to this B cell.
Remember, there might be other-- in fact, there
definitely are tons of other B cells around, but their
variable portions are all different.
And that's what I always find amazing about B cells is that
they come from the same genetic line, their genes have
been shuffled around in their development so that they can
produce billions of combinations of these
proteins, or the variable ends of their antibodies.
So let's say we have some virus.
Let me say it's a bacteria.
When I did the other B cell example, I said we're dealing
with viruses.
But let's say some new bacteria and just some part of
its surface just happens to bind to only this B cell--
because this B cell has just the right combination.
So some part of his surface binds just to that B cell
right there.
That part of the surface that binds, remember, that was
called the epitope.
That's that part of the pathogen that binds to our
variable sequencer.
It won't bind to this B cell or this B cell because they
have different sequences here, but it binds to this B cell
and then that starts the activation process.
We'll go into-- sometimes this by itself can get the B cell
activated, but you normally need help from helper T
cells-- and we'll talk more about that.
And we said, once this happens, once you get
activated or the activation process starts, this guy
actually gets engulfed-- and I didn't talk about that in the
last video just because I didn't want to go into too
much detail.
So this whole thing gets engulfed by the B cell.
And then when it gets activated, it proliferates
itself and you normally need the T cells there and some
part of them become plasma B cells, some part of them
become memory B cells.
Remember, the plasma B cells say, gee, I've been activated.
I'm just going to produce a ton of these antibodies.
So the plasma B cell will just produce tons of these
antibodies and start spitting them out so that they can
attach to more and more of that pathogen and just mess
them up in different ways, either tag them so that other
macrophages or phagocytes eat them up-- or tag two of them
so you bundle them up so that they can't operate properly.
Whatever.
I'm not going to go into detail on that.
That's when it gets activated.
But the interesting thing is that the B cell will also do
what the phagocytes do.
The B cell will also take this guy into the cell.
Maybe he's initially attached to the antibody-- and break
him up, take pieces of this pathogen and attach it to MHC
II proteins and then present them on the surface.
So a B cell will also present the antigen.
So this is also an MHC II complex-- a major
histocompatibility complex-- and just so you know, histo
means tissue.
So this is related to whether something is compatible with
the tissue in your body and we'll talk more about that and
how it relates to transplants and all of that.
So this is also an MHC class II.
In both cases, whether we're talking about B cells that
recognize a very specific pathogen-- and it could be a
specific virus, a specific protein, a specific bacteria,
or in the cases of phagocytes, they'll just say,
oh, you look shady.
Let me take you in.
I don't know what type of bacteria or virus or protein
you are, but in either case, they both engulf them, take
pieces of them, cut them up, and present them on their
surface in a complex with the major
histocompatibility complexes.
So cells that do this are called professional antigen
presenting cells.
This is what they do for a living, although they do other
things, as we've seen.
The phagocytes eat things.
The B cells generate antibodies or memories so that
they can later be activated to generate antibodies, but these
are called professional antigen presenting cell.
And the antigen in question is this little piece of the
actual thing that you're trying to track, that little
piece-- the actual part of the pathogen, that's what the
antigen-- so it's presenting the antigen.
It's professional because it takes pathogens from in the
fluid of our system and then engulfs them, breaks them up,
and then presents them.
Now there are also nonprofessional antigen
presenting cells.
And in fact, most cells are this.
In fact, even these guys.
Actually, I'm going to wait for the next video.
I realize all of my videos are getting long.
So you're probably thinking, these guys in either case
engulf them, cut them up, present them,
what is it good for?
You'll see these MHC II, these are what are recognized by
helper T cells.
It'll all form part of the puzzle of how our immune
system works.
So in the next video, I'll talk about MHC I presenting
cells, which is pretty much all body cells.