Self/nonself discrimination

[FORSDYKE at QUCDN.QueensU.CA]

(Ephraim Fuchs) says:
>Please help me with this question:
>
>Since antigen presenting cells take in antigens randomly from their
>environment, how is their delivery of signal two regulated so as to
>achieve a self/nonself discrimination?

    Does an antigen-presenting cell take in antigens randomly? Surely, an
antigen has first to be labelled as "foreign" by reacting with an antibody
(either free or cell-bound)? The pre-existing repertoire of antibodies
then determines which antigens will be taken up. The antibody repertoire is not
random, so why should antigen-uptake be random?

                     Sincerely,  Don Forsdyke
                                 Discussion Leader. Bionet.immunology