In article <v01510100ac7230de035f@[128.173.187.15]>,
Klaus D. Elgert <kdelgert at vt.edu> wrote:
>Immunology netters:
>>Along the recent lines of posting questions asked, in this case, by a
>graduate student during an undergraduate immunology lecture on antigens. I
>told her to see me later in the week, so how about some help before she
>gets here. The jest of her two questions was something like this (I
>think). We have a B cell specific for antigen X. It captures antigen X by
>its antibody receptors. These antibody receptors are directed against a
>specific epitope. Question 1: Once the B cell internalizes antigen X, how
>does it display (or know to display) the same epitope (peptide) by its
>class II molecules to the appropriate Th cell TCR? Question 2: If it is
>the same epitope, how does this Th cell become primed to the same peptide,
>through macrophage peptide-class II complex presentation, so that it can
>help the appropriate anti-X B cell? (When T cells do not recognize the
>same epitopes that B cells do.) This B cell then undergoes differentiation
>into a plasma cell, secreting antibody that reacts to the capture epitope.
>
The answer to this is that the antibody epitope and the T cell epitope need
not be the same (in fact, they are generally distinct). The antibody
epitope is defined by the random recombination of the antibody genes and
can be just about anywhere on the antigen. The T cell epitopes are limited
by the antigen processing machinery, which is why B cells and macrophages can
display the same peptide/MHC complexes.
This separation of the T and B cell epitopes is best exemplified by the
"carrier effect". In order to get an good immune response to a hapten, it
is usually conjugated to a "carrier" protein (such as KLH or BSA) which
contains many potential T cell epitopes (a hapten cannot by itself be a T cell
epitope). The B cell internalizes the complex using a hapten-specific surface
Ig molecule, and presents carrier-derived epitopes on MHC Class II to T cells.
In the meantime, the T cells can be primed by macrophages or dendritic cells
which internalize the carrier/hapten nonspecifically, and the B cells can
receive help to differentiate into a hapten-specific plasma cell, undergo
class-switching, etc.
Hope that helps,
BioKen
--
Ken Frauwirth (MiSTie #33025) _ _
frauwirt at mendel.berkeley.edu |_) * |/ (_ |\ |
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DNRC Title: Chairman of Joint Commission on In-duh-vidual Affairs