The classical complement pathway plays a major role in innate immunity against infection. This pathway is triggered by C1, a multimolecular complex composed of the recognition protein C1q and two serine proteases, C1r and C1s. Following the C1q recognition, C1r is autoactivated, and in turn activates C1s, which cleaves C4 and C2, the C1 substrates. C1r acts by transforming the activation signal into an enzymic activity. C1r is a single-chain glycoprotein which can be cleaved into an A chain and a B chain upon activation. This protien recognizes full-length C1r chain and the cleaved B chain.