Abstract: Methods and cells or cell lines for identifying antibodies or fragments thereof that activate heparin-induced thrombocytopenia (HIT) are described. The methods comprise contacting a hematopoietic cell or cell line that comprises, in operative association, the platelet receptor FcyRIIA under the control of a suitable promoter, and a reporter construct comprising a reporter gene under the control of a promoter and transcription factor, which transcription factor is regulated downstream of the signaling cascade of activated FcyRIIA, with a test sample from a mammalian subject; a platelet factor 4 (PF4), a wild-type or variant of PF4 or a fragment thereof; and heparin; and detecting or measuring the level of reporter gene expression.