The mechanism of complement activation by PF4/heparin complexes includes the following steps: (A) heparin (polyanion) forms ULCs with PF4 (cation); (B) polyreactive natural IgM from plasma binds to PF4/heparin ULCs, and conformational changes in IgM result in binding of C1q, which leads to activation of the classical pathway of complement activation; and (C) IgM and complement-coated PF4/heparin complexes bind to B cells via complement receptor 2. See Figure 7 in the article by Khandelwal et al that begins on page 2431.