The RhD antigen is clinically the most important human blood group antigen involved in hemolytic disease of the newborn (HDN). Immunization against the human platelet antigen (HPA)-1a is the most common cause of neonatal alloimmune thrombocytopenia (NAIT). Recent studies suggest that the immunization pattern for NAIT is similar to that occurring in HDN. There is a significant correlation between immunization and the presence of HLA restricted T cells in both HPA and RhD. Immunization to RhD is associated with HLA-DQB1* 02:01, and in several studies, alloimmunization to platelet antigen HPA-1a is strongly associated with both HLA-DRB3*01:01 and HLA-DQB1*02:01. Severe HDN occur seldom in first born children, and recsent studies have found that the incidence of immunization to HPA-1a in primigravidae is lower than previously assumed (-25 %). This may indicate that the primary immune response is established in association with, or after the delivery, most probably due to feto-maternal bleeding, similar to HDN.Still there are differences. NAIT unlike HDN, more frequently occurs in primigravidae, and anti-HPA-1a is detected at an earlier stage than anti-RhD (week 20 vs week 30). The reason for this might be that the presence of GPIIIa on circulating syncytiotrophoblast (ST) cellular material from placenta, could be the source of HPA-1a alloantigen causing primary immunization of primigravidae early enough for anti-HPA-1a to cause fetal thrombocytopenia during a first pregnancy. The RhD protein is expressed only in human red blood cells, and can enter maternal circulation only by feto-maternal bleeding.