C1 is the first complement component in the cascade referred to as the classical pathway of complement. C1 binds to and is activated by antibodies bound to antigens (immune complexes) yielding a protease that initiates the cascade. C1 is actually a non-covalent complex of three different proteins (C1q, C1r and C1s) bound together in a calcium-dependent complex. Description: Human serum/plasma. Synonyms: C1 protein.