The monoclonal antibody 4 recognizes the cleaved human C3 fragment C3c. The complement system is an important factor in innate immunity. The third complement component, C3, is essential to the classical, alternative and lectin pathways of complement activation. The activation products of the complement cascade contain new epitopes that are not present in various natural components. The synthesis of C3 is tissue-specific and is regulated in response to a variety of stimulants. C3 is the most abundant protein in the complement system, and the serum protein level is about 1.3 mg/ml. Inherited C3 deficiency makes people prone to bacterial infections. C3 fragments are deposited in tissues at the site of antibody-mediated immunopathology. In ulcerative colitis and idiopathic chronic inflammatory bowel disease, the deposition of C3 in the diseased mucosa has been reported. The proteolysis of C3 convertase cleaves C3 into C3a and C3b. C3b attaches to the immune complex and is further cleaved into iC3b and C3f. iC3b is further processed into C3c and C3dg. Monoclonal antibody 4 recognizes conformational epitopes in C3c, C3b and iC3b.