H antigen

• Also known as substance H, H antigen is a precursor to each of the ABO blood group antigens, apparently present in all people except those with the Bombay Blood phenotype
• Histocompatibility antigen, a major factor in graft rejection. Even when Major Histocompatibility Complex genotype is perfectly matched, can cause slow rejection of a graft.
• * major H antigens "encode molecules that present foreign peptides to T cells"
• * minor H antigens "present polymorphic self peptides to T cells". Includes, e.g. the H-Y antigen
• a bacterial flagellar antigen