The proliferation of T cells from patients with AITP against GPIIIa peptides is dependent on HLA-class II molecules. Cultures of PBMCs from a representative patient (AITP9) were stimulated with dominant GPIIIa peptides 2, 47, 53, or 82, and class II restricted responses were blocked by addition of antibody specific for HLA-DR,-DQ, or -DP. For each peptide stimulus, the dashed horizontal lines denote the level of inhibition taken as significant (> 50%). Similar results were obtained with PBMCs from another 3 patients.