C1q is part of the C1 complex and this complex is the first complement component in the cascade referred to as the classical pathway of complement. C1 is actually a non-covalent assembly of three different proteins bound together in a calcium-dependent complex. C1q has six extended arms with domains at the end of each arm that bind to the Fc domains of immunoglobulins. Description: Human serum/plasma. Synonyms: C1q protein.