Thrombocytes from HLA-A1 and HLA-A2 negative donors were incubated with plasma obtained from respective HLA-positive donors. The plasma-treated thrombocytes were shown to acquire HLA specificity of the plasma as demonstrated by their ability to absorb lymphocytotoxic activity of monospecific anti-HLA sera. The data are consistent with the hypothesis that the expression of HLA antigens in cells of hematopoietic origin diminishes in the course of differentiation and maturation and that the expression of HLA antigens on thrombocytes is primarily due to absorption of these antigens from plasma.

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