The P₁H antigen and antibody

P₁H, a newly discovered compound antigen associated with both the ABO and P systems, occurs in approximately 7 percent of Natal (South African) blacks. The compound antigen is evident only when the red cells have exceptionally strong expression of both P₁ and H antigens, and it is apparently a dominant character. The antigen is thought to originate by steric rearrangement in the molecule, or to be the product of competition between P¹ and H gene transferases for the available paragloboside. The corresponding antibody, anti-P₁H, has been made by both P₂ and P₂ people and is a weak cold agglutinin. It is not adsorbed by red cells carrying either strongly expressed P₁ or H antigens, but is adsorbed by and eluted from P₁H+ red cells sensitized by anti-P₁H. The antibody is inhibited by P₁ but not by H, Lewis, or Sd^a substances. Immunohematology 1993;9:7.