PBMCs from patients with AITP that respond to GPIIIa peptides are predominantly of the helper phenotype. PBMCs from patients AITP10 (A) and AITP18 (B) were either left unstimulated in culture or incubated with GPIIIa peptides 2 or 53 that induce proliferative responses in these patients, before being stained for CD4 expression and the activation marker CD71. Results are shown gated on the CD3⁺ population.