Complement C1s is a serine protease and a key component of the C1 complex in the classical complement pathway. One C1q molecule binds to a tetramer composed of two C1r and two C1s proteases to form the intact C1 complex. C1s circulates as a zymogen until activated by proteolytic cleavage from activated C1r. Its essential function is to cleave complement components C4 and C2, thereby initiating the formation of the C3 convertase (C4b2a). This step is critical for triggering downstream immune effector functions, including opsonization, inflammation, and membrane attack complex formation.