Monoclonal antibody I3 / 15 recognizes the activated complement protein C3, which is a new epitope on C3b, iC3b and C3dg, which is not present in natural C3. Antibody I3 / 15 recognizes natural C3 after unfolding after activation or after SDS treatment. C3 is a 190 kD protein that plays an important role in the complement activation cascade. This molecule is essential for 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. After activation, C3 is cleaved into two biologically active fragments called C3a and C3b. C3a is a mediator of the local inflammatory process with inflammatory toxic properties. Therefore, it induces smooth muscle contraction, increases vascular permeability, and causes histamine to be released from mast cells and basophils. C3b attaches to immune complexes and is further cleaved into iC3b, C3c, C3dg and C3f. C3 fragments C3b, iC3b, C3dg and C3d are collectively referred to as activated C3 (action C3). The most important function of the C3 fragment is the interaction of the complement system with other immune cells. The activation products of the complement cascade contain new epitopes that are not present in various natural components. Monoclonal antibodies that detect new epitopes have been used to directly quantify the activation of different steps in the complement cascade.