So let's talk about the
effector functions from T cells.
And there are going to be two flavours
of T cells that we need to deal with.
There are cytotoxic T cells
and there are helper T cells.
And they will respond in different
ways to dealing with infectious agents.
So, the first thing we have to
deal with our infectious agents
that are intracellular.
we need to be able to kill the
cell that is harbouring them.
That's our only way to respond.
To kill an infected cell, we bring in
cytotoxic T lymphocytes, killer T cells.
They are identified on their
surface by a marker called CD8.
And I apologise for the immunologists,
who named these various markers.
CD stands for cluster
And just knowing the number doesn't
help you figure out what cell type it is.
You just have to memories it.
So cytotoxic T lymphocytes
or killer T cells are also CD8+.
So that's how we recognise them.
So that CD8+ T cell needs to
somehow recognise that that
orange blob on
the left hand side
has become infected
with a virus and to kill it.
How does it do that?
Well, that blob on the left,
any any cell in the body,
is constantly surveilling
its internal environment.
It's constantly breaking
down proteins into peptides,
and then presenting
them out on their surface.
If everything that's inside the cell is
normal, and is self, we leave it alone.
However, if there's an invading
pathogen, an intracellular virus,
those viral proteins will be
broken down into little peptides,
and they're presented on the
surface of that cell via MHC class 1.
MHC stands for major histocompatibility
complex, and it's class 1.
It's a specific kind of
the MHC molecules.
So that infected orange blob
is now expressing on surface
MHC class 1 a normal molecule,
but in it, bound to it, is a little tiny
peptide that is derived from the virus.
And now specific cytotoxic T
lymphocytes can recognise it.
And when they do, what they do
is they release in a vectorial way
vacuoles that contain
granzyme and perforin.
The perforin punches holes in the target
cell and the granzymes induce a proptosis.
So, the goal here is that we need to be
able to recognise intracellular pathogens.
We do so via the generation
of peptides bound to class 1
that can be recognised by CD8
cytotoxic T lymphocytes that will kill.
Great. Now we've identified a way
to deal with intracellular pathogens.
That's on the right hand side.
In this particular case, there
are cells that run around the body,
macrophages, dendritic cells,
some endothelial cells a
variety of antigen presenting cells.
So their job is to surveil
the extracellular space
and look for pathogens, look
for microbes, look for invaders,
and they're constantly surveilling
the extracellular environment.
Everything that they
swallow, gulp, eat,
they will process into peptides
that they will express on their surface
with MHC class II.
So major histocompatibility
complex class two molecules.
And those little peptides
bound to MHC class II
potentially allow helper
T cells to recognise it.
If the outside world doesn't
contain any new molecules,
no new microbes, then all the
proteins that are presented ourself,
and we leave them alone.
However, if there's a microbe out
there, a bacterium or fungus or whatever,
the antigen presenting cell
will gobble that up, sample it,
put the peptide on MHC
class two and display it
so that helper T
cells can come along.
So remember we said there
are two flavours of T cells:
the cytotoxic T lymphocytes
on the left hand side,
the helper T cells in
the right hand side.
The marker for helper T
cells or TH cells is CD4.
That's a way that we can tell when
we do even a histochemical staining
or flow cytometry, which
population we're talking about.
Now the helper T cells recognise
this newly processed peptide
in association with MHC
class II and will respond.
Now we don't want
to kill the messenger.
We don't want to kill the
antigen presenting cell.
cell just saying,
"Hey dude, there's
infection going on out here."
So, we want to have the T cell
respond when it sees that process peptide
plus class II, we want
it to make cytokines,
that will bring in other
So, the cytokines that are
elaborated by that helper T cells
are going to coordinate, regulate,
activate macrophages and neutrophils
and other elements of the immune system
to go after that extracellular pathogen.
So two different flavours: killer T
cells - recognising peptide plus class 1
on target cells
and killing them.
And helper T cells CD4+,
that are recognising extracellular
peptide on edge of presenting cells
and then orchestrating a response
through cytokine production.
Cytotoxic T lymphocytes.
So they are recognising
plus MHC class 1 and will kill.
And that's their
major job in life,
but they also will do additional
things such as making cytokines.
Important point about the cytotoxic
T lymphocyte is its specific killing.
Only the target will die as a
result of the T cell interacting with it.
As opposed to
the helper T cells.
So the helper T cells recognising
extracellular pathogen peptide,
in association with MHC class
II on an antigen presenting cell
make a whole
variety of cytokines.
Those cytokines, however, just turn
on the macrophages and neutrophils.
They're very potent,
but they're pretty dumb.
And potentially once they get turned on,
turn all their activities towards itself.
So you can have a lot of
innocent by standard damage.
So the cytokines produced in this process
will activate macrophages and neutrophils
will recruit those effectors
but there's potentially important
nonspecific killing that's going on.