Complement
When the recognition function of the humoral immune system has been carried out through the production of speci?c antibodies and their binding to foreign antigens destruction of the invading pathogen is the next step. One major defense pathway is the complement system that is activated through antibodies bound to the invading microorganism and eventually causes it to lyse through punching holes in its plasma membrane.
The complement system consists of about 20 interacting soluble proteins in which extracellular ?uid and circulates in the blood. Immunoglobulin molecules bound to the surface of the microorganisms activate C1, the ?rst element of the complement pathway. The activation happens by the Fc portion of the bound antibody. Just bound antibody can activate complement; soluble antibody not bound to an antigen has no such that effect.
The early components of the complement pathway, involving C1, are proteases which activate their substrate through limited cleavage. Activated C1 now activates various molecules of the next elements through proteolysis, every of that activates various molecules of the next component through proteolysis, and so on. thus, the early steps in complement activation consist of a proteolytic cascade in that more and more molecules are activated at every step. An Component C3 is the key component whose cleavage leads to the assembly of membrane attack complexes on the plasma membrane of the microorganisms that makes holes in the plasma membrane which lead to cell death. Several white blood cells also become activated during this procedure and phagocytose the pathogen.
This pathway of complement activation, which starts when antibody has bound to antigen on the microbe surface, is called the classical pathway. An alternative pathway of activation also exists which is activated directly by polysaccharides in the cell wall of microorganisms even in the absence of antibody. The alternative pathway therefore defends the body against attack in the early stages before an immune response can occur and also augments the effects of the classical pathway of complement activation when the immune response has occurred.