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
