Complement C1r subcomponent is a ~80 kDa protein. 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. 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. Immunoglobulin-binding promotes the autocatalytic cleavage and activation of C1R. Activated C1R then cleaves and activates C1S, the second protease of the classical complement pathway. It is unclear if C1R activates C1S within single, strained C1 complexes or between neighboring C1 complexes on surfaces. 1. Kardos, J. et al. (2008) Molecular immunology 45, 1752-60. PMID: 179969452. Bally, I. et al. (2009) The Journal of biological chemistry 284, 19340-8. PMID: 194739743. Ugurlar, D. et al. (2018) Science (New York, N.Y.) 359, 794-797. PMID: 294494924. Zwarthoff, SA. et al. (2021) Proceedings of the National Academy of Sciences of the United States of America 118. PMID: 341551155. Lacroix, M. et al. (2001) The Journal of biological chemistry 276, 36233-40. PMID: 114455896. Kardos, J. et al. (2001) Journal of immunology (Baltimore, Md. : 1950) 167, 5202-8. PMID: 116735337. Major, B. et al. (2010) The Journal of biological chemistry 285, 11863-9. PMID: 201789908. Arlaud, GJ. et al. (1980) Biochimica et biophysica acta 616, 116-29. PMID: 62545709. McRae, BJ. et al. (1981) The Journal of biological chemistry 256, 12362-6. PMID: 6271784 | 
