Abstract
Well-defined proteolysis of complement component 3 (C3) produces fragments that mediate its classic activities including target opsonization and phagocytosis. In this issue of The EMBO Journal, Demir et al, (2025) uncover a novel C3 fragment, C3-LHF1, in the plasma of individuals with systemic lupus erythematosus (SLE), that curbs complement activation, modulates IL6ST signalling, and correlates with disease regression.