What is Complement system in Immunology?

When the recognition function of the humoral immune system has been carried out by the production of specific antibodies and their binding to foreign antigens, destruction of the invading pathogen is the next step. One main defense pathway is the complement system which is activated by antibodies bound to the invading microorganism and eventually causes it to lyse by punching holes in its plasma membrane. The complement system consists of about 20 interacting soluble proteins that circulate in the blood and extracellular fluid. Immunoglobulin molecules bound to the surface of the microorganisms activate C1, the first component of the complement pathway. The activation occurs through the Fc portion of the bound antibody. Only bound antibody can activate complement, soluble antibody not bound to an antigen has no such effect. The early components of the complement pathway, including C1, are proteases that activate their substrate by limited cleavage. Activated C1 now activates several molecules of the next component by proteolysis, each of which activates several molecules of the next component by proteolysis, and so on. Therefore, the early steps in complement activation consist of a proteolytic cascade in which more and more molecules are activated at each step. Component C3 is the key component whose cleavage leads to the assembly of membrane attack complexes on the plasma membrane of the microorganisms, which create holes in the plasma membrane that lead to cell death. Various white blood cells also become activated during this process and phagocytose the pathogen.