Complement component 3, often simply called C3, is a protein of the immune system that is found primarily in the blood. It plays a central role in the complement system of vertebrate animals and contributes to innate immunity. In humans it is encoded on chromosome 19 by a gene called C3.[5][6]
Deficiencies and defects of C3 result in the affected person being immunocompromised and particularly vulnerable to bacterial infections.