IMMUNOREACTIONS INVOLVING PLATELETS. VI. REACTIONS OF MATERNAL ISOANTIBODIES RESPONSIBLE FOR NEONATAL PURPURA. DIFFERENTIATION OF A SECOND PLATELET ANTIGEN SYSTEM*

Abstract

Maternal immunization against antigens on fetal platelets was found to be the basis of megakaryocytic or amegakaryocytic neonatal thrombocytopenic purpura in 6 children born of 4 normal mothers. In 2 families there was mother-fetal incompatibility with respect to a previously described platelet antigen, PlAl; and in the other families, with respect to a different antigen which was labeled PlBl. The PlBl antigen was inherited as a dominant character, was not an allele of PlAl, and differed from erythrocyte antigens. There appeared to be a proportionality between gene dose and quantity of PlBl antigen per cell. The frequency of reactors with anti-PlBl in the general population was approximately 39%. Complement fixation proved to be the only serologic technique by which the maternal antibodies could be detected. One of the antibodies could be measured only by its ability to block the reactions of a known complement-fixing anti-PlAl antibody. The PlBl antigen subsequently has been found to be shared by granulocytes and lymphocytes.