This pre-immunization was suggested to be caused by bacterial infections, supported by data on charge-dependent binding of PF4 to various bacteria, cross reaction of human heparin-induced anti-PF4–heparin antibodies with PF4-coated Staphylococcus aureus and Escherichia coli, and the appearance of anti-PF4/heparin antibodies during bacterial sepsis without heparin application in mice. This evidence concerns the gene PF4 and bacterial infectious disease.