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Complement system
Medical expert of the article
Last reviewed: 05.07.2025
The complement system consists of 9 sequentially activated components and 3 inhibitors. This system plays an important role, especially in inflammation and in the development of the body's resistance to infectious agents. In order for a bacterial or other cell to be lysed, activation of the complement system components from C3 to C9 is necessary via the classical or alternative pathway. The complement system is of great importance not only in cytolysis processes, but also in enhancing phagocytosis, neutralizing viruses, and in immune adhesion, due to which antigen-antibody complexes are attached to some cells, including B-lymphocytes.
Defects in the complement system are accompanied by a decrease in the body's anti-infective resistance.
Simultaneous determination of 3 indicators - C3, C4 components and complement activity titer allows to evaluate the state of both classical and alternative activation pathways. Complement consumption via the classical pathway (immune complexes) is accompanied by a decrease in all three indicators. When complement is activated via the alternative pathway (for example, in glomerulonephritis), C3 and complement activity titer are reduced, and C4 (a component of the classical cascade) remains normal. Determination of complement activity titer is a good screening method for complement deficiency (characterizes the presence of all components of the complement activation pathway). A reduced or undetectable level of complement activity titer indicates a hereditary deficiency of the complement system.