Is also an antigenic target in APS. Abs to PF4 are implicated in heparin-induced thrombocytopenia but have also been detected in patients with APS without a history of heparin-induced thrombocytopenia or recent exposure to heparin. Levels of anti-PF4 Abs also correlate with levels of IgM aPL and IgM anti-2GPI Abs, but have activity independent of both [55,56]. These data suggest another potentiall