Complement Component C1

**Semantic type:** Amino Acid, Peptide, or Protein

**Definition:** A protein complex composed of the following complement component 1 (C1) moieties, two C1r chains, two C1s chains and one C1q complex. C1 plays a role in the activation of the classical complement pathway. Binding of pentraxins or antibody-antigen complexes containing either immunoglobulin M (IgM) or several monomeric IgG molecules to the C1q complex moiety causes confirmational changes in C1q that activate the C1r chains. In turn, active C1r cleaves the C1s chains. Then cleaved C1s proteolytically activates C2 and C4 and promotes formation of C3-convertase, which can both cleave C3 and activate the classical complement pathway.

**Synonyms:** - C1 - C1 Complex - Complement C1 - Complement Component C1 Complex - Complement System C1 Complex