COMPLEMENT-FIXING PLATELET ISO-ANTIBODIES .3. QUANTITATION OF HL-A2 ANTIGEN ON PLATELETS

Summary. The amount of one of the human transplantation antigens, HL‐A2, was estimated in platelets from 69 HL‐A2‐positive individuals using titrations of platelets and complement fixation with constant amounts of two sera containing anti‐HL‐A2. The HL‐A2 zygosity was deduced from HL‐A typing and HL‐A chromosome segregation in 50 individuals belonging to 13 families. Within these families, HL‐A2‐homozygous individuals had, on an average, twice as much HL‐A2 antigen on their platelets as heterozygous individuals. There was about a two‐fold variation in the amount of HL‐A2 antigen on platelets from HL‐A2‐heterozygous individuals, whether they were related or not. Evidence is presented that part of this variation is due to the amount of HL‐A2 antigen being influenced by the simultaneous presence on the platelets of certain other HL‐A antigens controlled by genes belonging to the same series of mutually exclusive genes as the HL‐A2 gene. © 1969 Blackwell Publishing Ltd