Rejection of skin grafts between MHC identical mice and the development of severe graft versus host disease in bone marrow transplantation between MHC identical siblings imply the existence of structures other than MHC that can be recognised by T cells to activate the immune response. It is felt that the minor histocompatibily complex (MiHC) consists of small endogenous peptides that occupy the binding sites of self-MHC molecules and can trigger T cell responses between MHC identical individuals. Whereas MHC antigens can be identified by both B and T lymphocytes, repsonse to MiHC antigens appear to be strictly T cell mediated.
One well recognised minor antigen is encoded on the Y chromosome. This H-Y antigen is foreign to female so may cause recjection when a male kidney is transplanted into a female recipient. Another troublesome minor antigen is the monocyte/endothelial antigen series, whish may cause rejection in both renal and cardiac transplantion.