Studies on the binding of immunoglobulins and immune complexes to the surface of human platelets: IgG molecules react with platelet Fc receptors with the CH3 domain.

Suspensions of human platelets were incubated with various immunoglobulin preparations and subsequently stained with FITC-conjugated antisera. Incubation with monomeric, IgG, but not with monomeric IgM, IgA, IgD nor with IgE, gave a positive staining of the platelets. Incubation of platelets with monomeric IgG1 and IgG3 as well as with Fc and pFc'-fragments from IgG3 also gave a positive staining while incubation with monomeric IgG2 and IgG4 did not. Thus, IgG binds to Fc receptors on the surface of human platelets with the CH3 domain of the Fc region. Heat aggregation also caused binding of IgG2 but not with IgG4 proteins to human platelet Fc receptors. The majority of platelet preparations both from ITP and SLE patients gave a positive staining using direct immunofluorescence technique. Incubation of normal human platelets with sera from ITP and SLE patients gave a strong surface staining of normal platelets. Strong staining was also obtained with pepsin-digested sera from most patients with ITP while pepsin-digested sera from patients with SLE did not give a positive staining. It is therefore concluded that the majority of sera from patients with ITP contain antibodies with specificities for platelet surface antigens while sera from patients with SLE contain immune complexes that react with platelet Fc receptors through the Fc parts of the IgG antibodies.