Very quickly, we'll talk
about polyclonal activation.
So, there is a form of
type four hypersensitivity
that is related to proteins that
are elaborated by certain microbes,
this will lead to polyclonal activation,
these are the so called super antigens.
What is shown here on the slide
is the surface of an
antigen-presenting cell (APC),
it has on its surface
MHC class II,
and it has been processing
happily all the proteins around it,
and taking peptides and putting
it in to its new class two molecule.
And then it
presents it to T cells.
And the T cells have their receptors
formed of alpha and beta components
that are going to recognise
the peptide and MHC class II.
And if all of the peptides in the MHC
class II, ourself, then we let them go,
and if they are recognised as foreign,
if they come from a foreign object,
that microbe or whatever, then we
will elicit a helper T cell response.
Well, there's a certain class of
molecules called superantigens.
Normally elaborated by bacteria,
these super antigens bind
outside of the peptide groove,
and bind independently
of any particular peptide
that might be associated
with MHC class II.
And what they will do is will bind to a
particular subset of the V beta elements,
so the variable region, beta
chain of the T cell receptor,
and it will cross link all
of the class to on APCs
and all of the V betas of that
particular kind on every T cell.
Well, because T cells only have maybe six
or seven V beta chains to choose from.,
what that means is if you make a
superantigen, dump it into the system,
you're going to get
activation of somewhere
between 10 to 20%
of all the T cells,
but that superantigen
glueing everything together.
So as a result, the T cells
are cranking out now cytokines
but you're not just having a
little bit here a little bit there,
you're having a lot everywhere.
20% of the T cells firing all at once,
and you get a resulting cytokine storm.
Examples of this are toxic shock syndrome,
associated with Staphylococcus aureus,
and staph scalded skin syndrome,
all because those bacteria
make a super antigen that can fit
outside the antigen
and can activate a whole
bunch of T cells all at once.
So another form of
T cell mediated injury.