Complement-containing PF4/heparin complexes bind to CR2 (CD21) on B cells. (A) Binding of PF4/heparin to peripheral blood B cells correlates with CD21 expression. Whole blood was incubated with PF4/heparin and stained with labeled antibodies to CD19, CD21, and KKO. Top panel: gating of B cells based on CD21 expression; bottom panel: overlay histogram of KKO staining as a function of CD21 expression as low- (open curve with solid line) or high- (shaded area) expressing B cells. (B) CD21 mediates binding of complement-coated PF4/heparin complexes to B cells. Blood was incubated with anti-CD21 or control IgG before the addition of PF4/heparin. Top panel: a representative overlay histogram is shown for KKO staining of B cells with anti-CD21 or control IgG. Bottom panel: summary of results (mean ± SD) from 3 experiments for PF4/heparin binding to B cells after expression of CD21 was blocked with a polyclonal anti-CD21 antibody. **P < .005.