In the body, in addition to special cells that protect against microbes, there is another mechanism of nonspecific defense. That is, not against a specific pathogen, but against any foreign agents.

This method of fighting diseases is carried out by the "complement system" - a special group of proteins (about 20 in total), which are interconnected with each other.

Each protein is designated by the letter C and the corresponding number - C1, C2, C3, C4... Activation of one component leads to activation of others. Then from separate activated fragments (C1, C8...) an active complex (C2C3C4) is formed, which already has a biological effect. The complement system is part of innate and acquired immunity. The action of this system is very strong, so it is in a "disassembled" state so as not to harm the body.

C3 triggers a chemical reaction that helps immune cells to "swallow" and destroy the microorganism (opsonization). C4 triggers another defense mechanism, which results in a "breach" in the microbe's envelope that allows extracellular fluid to enter, after which the microorganism is killed. The most informative is the determination of both complement components .