Platelet alloantigens and other surface markers were studied in platelet cohorts of different mean density, using monoclonal and polyclonal probes. High density (HD) platelets expressed 12% more P1A1 molecules (46,942) than low density (LD) platelets (41,892). However, LD platelets carried 42% more HLA-A2 molecules (6,267 +/- 184) than HD platelets (4,406 +/- 232) (P less than .01) and 55% more class I HLA antigens (17,034 +/- 2,062 v 11,007 +/- 2,190) (P = .05). The platelet subpopulations did not differ in their content of glycoprotein (GP)IIb/IIIa complex or Baka antigen. The difference in expression of class I HLA antigens on HD and LD platelets is consistent with two possibilities: either class I HLA molecules are acquired from plasma or they are released into plasma as platelets age in circulation. Accordingly, class I HLA molecules may provide a useful marker of platelet age.

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