Polyreactive IgM mediates complement activation by PF4/heparin. (A) Antigen-specificity of commercial IgM was determined using microtiter plates coated with various antigens. The graph shows the binding (y-axis) of various concentrations of commercial IgM (1.25-80.0 µg/mL) to different antigens. (B) Complement activation by polyreactive monoclonal IgM, 2E4, in the plasma of 2 donors (low complement activation phenotype; circle/square) in response to buffer (open symbols), PF4 alone (hatched symbols), heparin alone (half-filled symbols) and PF4/heparin (filled symbols) as measured by the antigen-C3c capture ELISA assay. Graph shows complement activation (y-axis) as a function of added polyreactive IgM concentrations. Results are shown from a representative experiment involving 3 donors tested on 3 different occasions. **P < .0001, relative to no polyreactive IgM added. (C) Whole blood from the cord blood of a baby was incubated with buffer or PF4 ± heparin and binding of PF4/heparin (KKO) or C3c to the B cells was determined by flow cytometry. Histograms show the representative results from 2 different experiments with 2 different cord blood samples.