Complement activation and its pathways in primary immune thrombocytopenia.

Complement activation has been increasingly recognized in immune thrombocytopenia (ITP). However, the involvement of alternative pathways in ITP and relevance of complement activation in refractory ITP remain unclear. This multicenter observational study aimed to characterize complement activation in primary ITP (pITP). C3a des-Arg levels were significantly higher in patients with pITP than in patients with systemic lupus erythematosus/antiphospholipid syndrome and healthy donors (HD). The expression of Bb, an alternative pathway-specific marker, did not differ between groups. sC5b-9 levels were significantly elevated in patients with pITP compared to those in HD patients. Patients with a partial response to pITP who met the criteria for refractory ITP with repeated relapses despite standard treatments exhibited elevated levels of C3a des-Arg and sC5b-9. Complement activation was observed in pITP, whereas involvement of the alternative pathway was limited. These findings highlight the importance of complement assessment in patients with pITP and may guide the optimal use of complement-targeted therapies.