The immune system has evolved several mechanisms that provide lymphocytes w
ith the intelligence to ignore self proteins while attacking foreign pathog
enic agents. Notably, B and T lymphocytes that encounter self antigen at ei
ther the inappropriate levels or affinity are usually instructed to perish
or become anergized. However, the presence of autoimmune disease suggests t
hat the induction of self tolerance is not foolproof. In fact, autoreactive
cells are now found to be normal inhabitants of the B and T lymphocyte rep
ertoire. This review examines how foreign peptides which resemble self prot
eins can elicit autoimmunity that is amplified to many sites on a target au
toantigen. In particular, B lymphocytes initiated by foreign molecular mimi
cs can process and present self peptides in the shaping of autoimmune T cel
l responses.