Monoclonal antibody 755 recognizes an epitope located in the C-terminal 360 amino acids on the α chain of C3, thereby recognizing C3b and intact C3. 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 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. After the complement system is activated, certain enzymes become active, leading to the cleavage of C3 into C3b and anaphylatoxin C3a. C3b attaches to immune complexes and is further cleaved into iC3b, C3c, C3dg and C3f. In the alternative pathway of complement, C3b plays a key role in the amplification loop triggered by the spontaneous hydrolysis of C3.