PBMCs from patients with AITP proliferate in response to peptides from the sequence of GPIIIa. PBMCs were isolated from representative patients AITP1 (A), AITP8 (B), AITP10 (C), and AITP20 (D) tested for the ability to proliferate against the panel of 86 peptides spanning the GPIIIa molecule. The dashed horizontal line denotes the level of proliferation taken as representing a significant positive response (SI > 3).