Review: platelet immunology—similarities and differences with red cell immunology

In many ways platelet and red blood cell (RBC) immunology are similar, but there are important differences. At least 272 RBC antigens have been described. These have been placed into 22 systems (194 antigens), 7 collections (20 antigens), and 2 series (high-frequency antigens [12] and low-frequency antigens [46])¹. Platelets express human leukocyte antigens (HLA) (class 1 only), ABH, P (not P₁), Lewis, I, and platelet-specific antigens on their membranes. Platelet-specific antigens can be classified into five biallelic systems, originally called Pl^A1 (Zw), Ko(Sib), Bak, Yuk (Pen), and Br. Each system includes a high-frequency antigen occurring in 96 percent to >99 percent of the population, and homozygosity for a lower-frequency antigen, occurring in 4 percent to <1 percent of the population. In addition, a number of additional low-frequency (private) antigens (e.g., Mo^a, Ca^a, Sr^a, Va^a, Iy^a, Pe^a, Gro^a, Max^a) have been described.²⁻⁴ Like RBC antigens and antibodies, platelet-specific antigens and antibodies were first named using letters from the name of the propositus (e.g., Bak, Br, Yuk).