Self vs. non self

[Jay and Nancy Mone]

The short answer goes like this:

During T cell ontogeny, T cells undergo selection in the thymus.  As 
thymic cells present self antigens to young T cells, any T cell which can 
respond to those antigens begins down a biochemical pathway that leads to 
cell death.  In this way, autoimmune responses are avoided without 
compromising the ability to respond to foreign antigens.  What is meant 
by self antigen is an antigen produced by your own body.  Since no T 
cells are present which can react to self antigens, these antigens are 
tolerated (hence the name tolerance).

Jay M