In the circulation, platelets have the greatest amount of circulating HLA class I molecules and most of these HLA molecules are adsorbed on the platelets. The majority of HLA antigens on the platelet surface are composed primarily of heavy chains; therefore, they have the ability to dissociate from the platelet especially with storage. When platelets are transfused into an allogenic recipient, the host is thus exposed to a huge dose of potentially altered donor HLA class I moleculles. The donor HLA molecules (whether still associated with the platelet or not) eventually circulate through the spleen and are either phagocytosed (eg the platelet-associated class I) or taken up by pinocytosis (soluble class I) by cells (eg macrophages) of the reticuloendothelial system. These initial uptake mechanisms allow for recipient splenic macrophages .