Observations on the distribution of the Zwa (P1A1) antigen on fixed and unfixed platelets

We have investigated the distribution of the Zwa antigen on unfixed platelets and platelets fixed in paraformaldehyde by examining thin sections of platelets by electron microscopy. Fixation of the platelets produces an even distribution of antigen around the surface membrane of the platelets. Unfixed platelets show clustering in some areas with other areas apparently free of antigen. In addition to this clustered antigen distribution, the unfixed platelets also show invaginations and vesicles containing antigen. The significance of these findings is discussed.