C1

First component of complement; actually three subcomponents, C1q, C1r and C1s, that form a complex in the presence of calcium ions. C1q, the recognition subunit, has an unusual structure of collagen-like triple helices forming a stalk for its Ig-binding globular heads. Upon binding to immune complexes the C1 complex becomes an active protease that cleaves and activates C4 and C2.