Complement activation by PF4/heparin correlates with plasma/serum IgM levels. (A) Proteomic analysis of plasmas with a high, intermediate, or low complement response phenotype shows strong correlation with plasma IgM. Graph shows IgM protein intensity determined by proteomic analysis (x-axis) and complement activation response to PF4/heparin, as measured by the antigen-C3c capture ELISA assay (y-axis). (B-D) Serum immunoglobulin levels (IgM, IgG, and IgA) from 29 healthy donors were measured in the clinical laboratory and correlated with an individual’s complement activation response to PF4/heparin, as measured by the antigen-C3c capture ELISA assay. Graphs show correlation of complement activation (y-axis) as a function of immunoglobulin levels (x-axis). Each symbol in the graph represents an individual donor. Complement activation values were normalized to an intermediate donor studied in parallel.