The function of C1S (complement C1s, Ensembl gene identifier ENSG00000182326) is as follows. 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:11445589, PubMed:16169853, PubMed:417728, PubMed:467643, PubMed:6271784, PubMed:6282646, PubMed:6319179, PubMed:70787, PubMed:9422791). C1S is activated following association of the C1 complex with immunoglobulins (IgG or IgM) complexed with antigens to form antigen-antibody complexes on the surface of pathogens (PubMed:34155115). C1S is cleaved and activated by C1R to generate C1s subcomponent heavy and light chains (PubMed:11445589, PubMed:6271784). C1s subcomponent light chain then cleaves and activates C2 and C4, the next components of the classical complement pathway (PubMed:16169853, PubMed:467643, PubMed:6282646, PubMed:6319179, PubMed:6906228, PubMed:70787, PubMed:9422791). {ECO:0000269|PubMed:11445589, ECO:0000269|PubMed:16169853, ECO:0000269|PubMed:34155115, ECO:0000269|PubMed:417728, ECO:0000269|PubMed:467643, ECO:0000269|PubMed:6271784, ECO:0000269|PubMed:6282646, ECO:0000269|PubMed:6319179, ECO:0000269|PubMed:6906228, ECO:0000269|PubMed:70787, ECO:0000269|PubMed:9422791}. [Complement C1s subcomponent light chain]: Serine protease component of the complement C1 complex, which catalyzes cleavage and activation of C2 and C4, the next components of the classical complement pathway (PubMed:16169853, PubMed:22949645, PubMed:417728, PubMed:467643, PubMed:6282646, PubMed:6319179, PubMed:70787, PubMed:9422791). Also able to cleave C1 inhibitor (SERPING1) in vitro; additional evidence is however required to confirm this result in vivo (PubMed:16169853). Also cleaves IGFBP5 and thereby inhibits the trophic effects of IGF1 (PubMed:18930415). {ECO:0000269|PubMed:16169853, ECO:0000269|PubMed:18930415, ECO:0000269|PubMed:22949645, ECO:0000269|PubMed:417728, ECO:0000269|PubMed:467643, ECO:0000269|PubMed:6282646, ECO:0000269|PubMed:6319179, ECO:0000269|PubMed:70787, ECO:0000269|PubMed:9422791}.