IL21 and autoimmune thrombocytopenic purpura: Finally, anti-complement 1q antibody (anti-C1q), complement factor H (CFH), complement fragments Bb (CFBb), stromal-derived factor-1 (SDF1, also known as CXCL12), and IL21 plasma levels were examined by Sahip et al. to see if there was any correlation between them and the clinical characteristics of ITP.