The present knowledge on chemical, enzymatic, serologic and genetic aspects of ABH antigens is
reviewed in an effort to produce a simple and coherent genetic model for the biosynthesis of these antigens and
chemically related structures. The genetic control of type 1 (Le^a, Le^b, Le^c and Le^d), type 2 (X, Y, I, and H), type 3 and
type 4 ABH and related antigens in different animal and human tissues is analyzed, taking into account the properties
of the glycosyltransferases which are involved in their synthesis and considering possible competition for common
acceptor and donor substrates. The phylogeny of ABH determinants shows that they appeared as tissular antigens
much earlier than as red cell antigens. The ontogeny of ABH antigens suggests that they behave as differentiation
antigens, and an effort is made to correlate their tissular distribution in the adult with the embryological origin of
each tissue.