Complement, in immunology, a complex system of more than 30 proteins that act in concert to help eliminate infectious microorganisms. Specifically, the complement system causes the lysis (bursting) of foreign and infected cells, the phagocytosis (ingestion) of foreign particles and cell debris, and the inflammation of surrounding tissue. The interacting proteins of the complement system, which are produced mainly by the liver, circulate in the blood and extracellular fluid, primarily in an inactivated state. Not until the system receives an appropriate signal are they activated. The signal sets off a chemical chain reaction in which one activated complement protein triggers the activation of the next complement protein in the sequence. Complement activation occurs by two routes, called the classical pathway and the alternative pathway, or properdin system. A different type of signal activates each pathway. The classical pathway is triggered by groups of antibodies bound to the surfaces of a microorganism, while the alternative pathway is spurred into action by molecules embedded in the surface membranes of invading microorganisms and does not require the presence of antibodies. Both pathways converge to activate the pivotal protein of the complement system, called C3. pathways of complement activationThe main function of complement proteins is to aid in the destruction of pathogens by piercing their outer membranes (cell lysis) or by making them more attractive to phagocytic cells such as macrophages (a process known as opsonization). Some complement components also promote inflammation by stimulating cells to release histamine and by attracting phagocytic cells to the site of infection. The activation of C3 fragments the protein into two pieces—a smaller piece, called C3a, which promotes an inflammatory reaction, and a larger piece, called C3b, which binds to the surface of a microbial cell. C3b helps bring about the elimination of the microbial invader in two ways: 1. Bound C3b activates the formation of membrane attack complexes, structures composed of other complement proteins that poke holes into the membrane of the invading microorganism and allow the contents of the cell to leak out and the cell to die. 2. The C3b-coated microorganism attracts white blood cells called macrophages and neutrophils and enhances their ability to ingest the microorganism or transport it to the liver or spleen for further processing. #SecondLineOfDefence #ComplementProteins #InnateImmuneResponse