Complement component 1s subcomponent serine protease

Complement component 1s (C1s) is a serine protease and a key enzymatic subunit of the C1 complex, which initiates the classical pathway of the complement system in humans. Upon activation by immune complexes (antigen-antibody), C1r activates C1s, which then cleaves complement proteins C4 and C2 to generate the classical pathway’s central enzyme, the classical pathway C3 convertase. Dysfunction or deficiency in complement component pathways—including mutations or deficiencies affecting *C1S*—are associated with immunodeficiencies such as systemic lupus erythematosus and hereditary angioedema; overactivation may contribute to autoimmune diseases and certain cancers. As such, it serves as both a diagnostic marker and potential therapeutic target.

Inhibition of C1s prevents cleavage of complement components C4 and C2, thereby blocking the classical complement pathway.