Anti-PF4/heparin antibodies are present in the normal population. Sera of participants (n = 4029) from the SHIP were tested for IgM- and IgG-binding to hu-PF4/heparin complexes using EIA as described in “P4/heparin antibody assays.” The results for anti-PF4/heparin IgG (A) and IgM (B) antibodies are shown. The distribution of antibody reactivities as measured by OD values represented a semilogarithmic continuum rather than 2 separate populations of immunized and nonimmunized persons. This is consistent with the concept that the general population is frequently exposed to PF4/heparin-like antigen complexes, which results in a continuum of the immune response. The OD values are arbitrary units. For comparison: with the same assay, strong reacting anti-PF4/heparin antibodies from patients with HIT reacted in an OD range of 1.0 to 2.0.