The function of C1R (complement C1r, Ensembl gene identifier ENSG00000159403) is as follows. Serine protease component of the complement C1 complex, a multiprotein complex that initiates the classical pathway of the complement system, a cascade of proteins that leads to phagocytosis and breakdown of pathogens and signaling that strengthens the adaptive immune system (PubMed:17996945, PubMed:19473974, PubMed:29449492). C1R catalyzes the first enzymatic step in the classical complement pathway: it is activated by the C1Q subcomplex of the C1 complex, which associates with IgG or IgM immunoglobulins complexed with antigens to form antigen-antibody complexes on the surface of pathogens (PubMed:29449492, PubMed:34155115). Immunoglobulin-binding promotes the autocatalytic cleavage and activation of C1R (PubMed:11445589, PubMed:11673533, PubMed:17996945, PubMed:20178990, PubMed:6254570, PubMed:6271784). Activated C1R then cleaves and activates C1S, the second protease of the classical complement pathway (PubMed:11445589, PubMed:11673533, PubMed:6271784). It is unclear if C1R activates C1S within single, strained C1 complexes or between neighboring C1 complexes on surfaces (PubMed:28104818, PubMed:29311313, PubMed:29449492). {ECO:0000269|PubMed:11445589, ECO:0000269|PubMed:11673533, ECO:0000269|PubMed:17996945, ECO:0000269|PubMed:19473974, ECO:0000269|PubMed:20178990, ECO:0000269|PubMed:28104818, ECO:0000269|PubMed:29311313, ECO:0000269|PubMed:29449492, ECO:0000269|PubMed:34155115, ECO:0000269|PubMed:6254570, ECO:0000269|PubMed:6271784}.