The complement system

The complement system consists of 9 consecutively activated components and 3 inhibitors. This system plays an important role, especially with inflammation and the development of resistance to infectious agents. In order for the lysis of a bacterial or other cell to occur, activation of the components of the complement from the C3 to C9 by the classical or alternative pathway is necessary. The complement system is of great importance not only in the processes of cytolysis, but also in the enhancement of phagocytosis, neutralization of viruses, as well as in immune adhesion, through which Ag-AT complexes are attached to certain cells, including B-lymphocytes.

Defects in the complement system are accompanied by a decrease in the anti-infective resistance of the organism.

Simultaneous determination of 3 indicators - C3, C4 components and the titer of complementary activity allows to evaluate the state of both classical and alternative ways of activation. The consumption of complement by the classical pathway( immune complexes) is accompanied by a decrease in all three indices. When complement is activated on an alternative pathway( for example, with glomerulonephritis), C3 and the titre of complementary activity are reduced, and C4( a component of the classical cascade) remains normal. The determination of the complementary activity titer is a good method of screening for complement deficiency( characterizes the presence of all components of the complement activation pathway).A reduced or undetectable level of the titer of complementary activity indicates a hereditary deficiency of the complement system.

Serum titre of complementary activity in serum

Normally, the titre of complementary serum activity in adults is 50-140 U / ml.

The complement titre in the serum assesses the activity of the terminal components of complement when it is activated by the classical and alternative routes.

Any inflammatory process with an adequate immune response is accompanied by an increase in the complement titer. Decreased titer indicates a lack of complement and leads to a weakening of its opsonizing function and complement-dependent cytotoxicity, which contributes to the accumulation of immune complexes and leads to chronic inflammation. The increase in complement activity is characteristic for allergic and autoimmune processes. In severe anaphylactic reactions, the complement titer decreases, and in case of anaphylactic shock it can not be detected at all.

Reference values ​​of C3 content in serum are 0.55-1.2 g / l.

C3 is the key complement component necessary for the realization of cytolysis. It is synthesized in the liver and is part of the resulting immune complexes. C3 is activated along the classical pathway by complexes Ag with IgG, IgM, in the alternative pathway - complexes Ar with IgA, IgE, Fab fragments Ig, polysaccharide Ag bacteria.

Reducing the concentration of the C3 component in the blood serum leads to a weakening of the opsonizing function of the blood, phagocytosis, cytolysis, and may be due to a violation of its synthesis or increased catabolism, and also its adsorption on immune complexes in autoimmune( immunocomplex) diseases. An increase in serum C3 concentration is characteristic of the acute period of infection( the "acute phase" protein).In the period of convalescence, the C3 concentration is normalized. The reference values ​​of the C4 content in blood serum are -0.2-0.5 g / l.

C4 is a component of the classic complement activation pathway. It is synthesized in the liver. Determination of its content is important for the diagnosis of immunocomplex diseases, in which it is adsorbed on immune complexes, which leads to a decrease in the amount of free C4 in the blood.