P1PK: The blood group system that changed its name and expanded

The antigens in the P1PK blood group system are carried on glycosphingolipids. The system currently includes three different antigens, P1, P^k, and NOR. The P1 antigen was disovered in 1927 by Landsteiner and Levine, and P^kand NOR were described in 1951 and 1982, respectively. As in the ABO system, naturally occurring antibodies of the immunoglobulin (Ig) M or IgG class, against the missing carbohydrate structures, can be present in the sera of people lacking the corresponding antigen. Anti-P1 is generally a weak and cold-reactive antibody not implicated in hemolytic transfusion reaction (HTR) or hemolytic disease of the fetus and newborn while P^kantibodies can cause HTR, and anti-NOR is regarded as a polyagglutinin. A higher frequency of miscarriage is seen in women with the rare phenotypes p, P₁^k, and P₂^k. Furthermore, the P^kand P1 antigens have wide tissue distributions and can act as host receptors for various pathogens and toxins. Why p individuals lack not only P^kand P expression but also P1 has been a longstanding enigma. Recently, it was shown that the same A4GALT-encoded galactosyltransferase synthesizes both the P1 and P^kantigens and that a polymorphism in a new exon in this gene predicts the P₁and P₂phenotypes. Immunohematology2013;29:25–33.