H antigen

H antigen can refer to one of the various types of antigens having diverse biological functions:
 * 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 (see hh blood group). The gene responsible for making H antigen is FUT1, located on the 19th chromosome in humans.
 * 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