Self/nonself discrimination

[Ephraim Fuchs]

Please help me with this question:

One of the cardinal features of the immune system is its ability to 
discriminate self from nonself.  Recent experimental evidence suggests 
that a two signal model of lymphocyte activation is the biological basis 
for S/NS discrimination.  For CD4+ T cells, signal two is delivered by 
the antigen presenting cell.

Here is the 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?