The complement system is a group of approximately 20 plasma proteins that strengthen the body's defenses against infections through opsonization, inflammation, and cell lysis. Opsonization involves coating pathogens with complement proteins, making them more recognizable and facilitating phagocyte engulfment. Certain complement proteins induce inflammation that attracts immune cells to the site of infection. Cell lysis involves the destruction of pathogens through the formation of a membrane attack complex (MAC).

These proteins, labeled C1 through C9, along with other regulatory proteins, usually circulate in an inactive state in the blood. The complement system can be activated in three ways: the classical, lectin, and alternative pathways. The classical pathway is the most rapid and effective mode of complement activation. It is activated when the C1 complex binds to antibodies that are attached to antigens on the surface of pathogens. In contrast, the lectin and alternative pathways are antibody-independent. The lectin pathway is triggered by the attachment of mannose-binding lectin proteins to pathogens. The alternative pathway, unique in its continuous low-level activity, is activated when complement proteins P, D, and B interact with specific molecules present on the surface of pathogens.

Regardless of the activation pathway, the complement system eventually leads to the cleavage of C3, a central component, into C3a and C3b. C3a plays a crucial role in promoting inflammation by attracting immune cells to the site of infection and enhancing vascular permeability. C3b, on the other hand, facilitates opsonization by binding to the surface of pathogens and marking them for destruction by phagocytosis. Additionally, C3b contributes to forming the MAC. This complex is formed by several complement proteins that assemble on the surface of the pathogen, creating pores in their cell membranes. These pores compromise the membrane integrity of the pathogen, leading to cell lysis and destruction.