Monoclonal antibody 3/26 recognizes the C3 fragments C3b, iC3b and C3c activated by mouse complement proteins. 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. C3 is the most abundant protein in the complement system, and the serum protein level is about 1.3 mg/ml. The proteolysis of certain enzymes causes the cleavage of C3 into C3a and C3b. C3b attaches to immune complexes and is further cleaved into iC3b, C3c, C3dg and C3f. 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. Hereditary C3 deficiency makes people vulnerable to frequent attacks from bacterial infections. In ulcerative colitis and idiopathic chronic inflammatory bowel disease, the deposition of C3 in the diseased mucosa has been reported. Monoclonal antibody 3/26 preferably recognizes cleaved C3 fragments C3b, iC3b and C3c. In the case of acute inflammation, many C3 are processed into products recognized by 3/26, so they can be used as markers of inflammation. Under chronic inflammatory conditions, the lowest reactivity with 3/26 can be observed. In this case, the C3dg product is mainly located in the inflammation (C3c is cleared) that the antibody (3/26) cannot recognize. The long-term processing/activation of C3 is at a low level, which will reduce the detection of C3 fragments C3b, iC3b and C3c.