The non-MHC transplantation antigens: neither weak nor minor.

The existence of minor histocompatibility (mH) antigens is implied by the very naming of the major histocompatibility antigens. Grafts exchanged between individuals matched at the major histocompatibility complex (MHC) (which encodes the antigens that produce rapid graft rejection) are rejected, albeit at a somewhat slower pace than MHC grafts. There are multiple mH antigen differences between MHC (HLA) matched humans and between MHC (H-2) identical mouse strains. Using mH congenic mouse strains, however, individual mH antigens can be studied, and these provide a basis for interpretation of immune responses to multiple mH antigens in HLA matched human grafts. In this article Bruce Loveland and Elizabeth Simpson discuss four aspects of mH antigens: their immunogenicity; immune response (Ir) gene regulation; mH antigens in man; and recent models which might lead to their molecular identity.