Activation of complement by PF4/heparin complexes and binding to B-cell CD21. (A) PF4 and heparin interact over a narrow range of molar ratios to generate ULCs. PF4 alone or PF4 with excess of heparin do not make ULCs. (B) ULCs activate complement and bind complement activation products (C3/C4). (C) Complement-coated ULCs bind to a B cell via CR2 (CD21).