Peptide/HLA complexes as targets of T cell–mediated alloimmune reactivity. (A) Self peptides expressed in self HLA molecules induce tolerance. (B) Polymorphic nonself peptides recognized in the context of self HLA molecules can induce alloreactivity; these polymorphic nonself peptides are called minor histocompatibility antigens (MiHA). (C) Monomorphic (self) peptides presented in the context of nonself HLA molecules provoke alloreactivity. (D) Polymorphic nonself peptides in the context of nonself HLA molecules can also provoke alloreactivity.