Abstract.In order to determine whether HLA‐A,B antigens of platelets are integral membrane constituents or rather represent adsorbed plasma proteins, their presence in plasma and their adsorbability onto platelet membranes was studied by in vitro and in vivo experiments. The amount of HLA antigens was quantitated by inhibition of lymphocytotoxicity (LCT) and by enzyme‐linked immunosorbent assay (ELISA) using operationally monospecific polyclonal HLA antibodies or murine HLA‐specific monoclonal antibodies, respectively. We found that in 11 out of 13 HLA‐A2 and in 9 out of 10 HLA‐B13 experiments, platelets from antigen‐negative donors pretreated with plasma from the same number of antigen‐positive donors inhibited LCT to the same extent as platelets from antigen‐positive donors. Nevertheless, the in vitro adsorbed HLA antigens onto antigen‐negative platelets were, unlike those on antigen‐positive platelets or in plasma, not reactive with monoclonal antibodies as quantitated by ELISA. Similarly, infusion of HLA‐A2‐negative platelets from single donors into 3 HLA‐A2‐positive, thrombocytopenic patients with bone marrow failure led to a good platelet increment, but did not convert the HLA type of donor platelets, neither at 2 h nor at 18 h posttransfusion. On the basis of these results, we conclude that soluble HLA antigens can be taken up by human platelets from plasma in small amounts. However, the major portion of HLA antigens appears to be int
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