The complement system consists of around 20 types of soluble proteins that eliminate extracellular infections. Continuous complement protein synthesis occurs in the liver and macrophages; these proteins are plentiful in the blood serum and can respond rapidly to invading pathogens.

The complement system is so named because it complements the adaptive immune system's antibody response. Complement proteins bind to the surfaces of bacteria and are especially attracted to pathogens already bound by antibodies.

Each successive complement protein is activated by cleavage and/or structural changes induced by the binding of the preceding complement protein (s).

After the initial few complement proteins bind, a cascade of sequential binding events ensues, resulting in a rapid coating of the pathogen with complement proteins.

Complement proteins serve multiple purposes. This process is known as opsonization. The proteins serve as a flag to indicate the presence of a pathogen to phagocytic cells, such as macrophages and B cells, and increase engulfment.

Opsonization refers to an immune process where particles such as bacteria are targeted for destruction by an immune cell known as a phagocyte. Certain complement proteins can combine to form attack complexes that open pores in microbial cell membranes. These structures destroy pathogens by causing their contents to leak

On the surface of a pathogen, the complement cascade can be activated by three distinct mechanisms. The classical pathway is begun by the binding of C1q to the surface of the pathogen or to antibody/antigen complexes, thereby establishing a link between innate and adaptive immunity.

A second activation mechanism is the mannan-binding lectin pathway (MB-lectin pathway). This is initiated by the binding of mannan-binding lectin, a serum protein, to mannose-containing carbohydrates on the pathogen. The alternate pathway is the last activation method. It begins when an active complement component attaches to the pathogen's surface.

Each pathway initiates processes that produce C3 convertase, the "early" events of complement activation. Pathogens covalently bind C3 convertase. C3b causes inflammation by cleaving C3. C3b directs phagocytes to kill pathogens.

It also binds C3 convertase to make C5 convertase, which produces C5a, a short peptide mediator of inflammation, and C5b, which starts "late" complement activation. Late-stage polymerization processes form a membrane-attack complex. Some bacteria and viruses have membrane attack complex pores.

Complement recognises and eliminates germs. Pathogens or antibodies activate the system. Regulatory proteins control complement system activity to prevent tissue injury from improper binding of activated component proteins.