Complements (Immunology)

Complement is a complex system of enzymes, regulatory proteins, cell surface receptors that are involved in host defense, inflammation, and modulation of immune responses. The system provides a fast-acting mechanism for the identification and removal of foreign substances, providing protection before the adaptive immune system can come into play. It is also involved in a wide variety of homeostatic processes including the clearance of immune complexes, effete cells,  cellular debris from damaged tissues etc.

Paul Ehrlich coined the term complement, defining it as “the activity of blood serum that completes the action of antibody”. Complement was discovered by Jules Bordet as a heat-labile component of normal plasma that causes the opsonization and killing of bacteria.


 Fig: Complements


It refers to a set of serum proteins that cooperates with both the innate and the adaptive immune systems to eliminate blood and tissue pathogens. Various complement components bind and opsonize bacteria, rendering them susceptible to receptor-mediated phagocytosis by macrophages, which express membrane receptors for complement proteins. Other complement proteins elicit inflammatory responses, interface with components of the adaptive immune system, clear immune complexes from the serum, and/or eliminate apoptotic cells. At last, a Membrane Attack Complex (MAC) assembled from complement proteins directly kills some pathogens by creating pores in microbial membranes.


Fig: Complement molecules


The complement system is a part of the immune system that enhances (complements) the ability of antibodies and phagocytic cells to clear microbes and damaged cells from an organism, promotes inflammation, and attacks the pathogen’s plasma membrane.


The complement system consists of serum, cell surface proteins that interact with one another and with other molecules of the immune system in a highly regulated manner to generate products that function to eliminate microbes. Complement system refers to a series of more than 20 plasma proteins, circulating in the blood and tissue fluids. Most of the proteins are normally inactive; they are activated only under particular conditions to generate products that mediate various effector functions of complement. Complement are activated in an enzyme cascade – the activation of one protein enzymatically cleaves and activates the next protein in the cascade



Complement components are designated by numerals, viz., C1–9. These components circulate in plasma in the form of proenzymes that are functionally inactive. Activation involves cleavage by proteolysis into peptide fragments. The fragments are designated with lowercase suffixes—for example, C3 is cleaved into two fragments, C3a and C3b. Normally, the large fragment is designated “b”, and the small fragment “a”. But for historical reasons, with respect to the fragments of C2, the large fragment is designated C2a and the small one is designated C2b.

Download link: Complement

Leave a Reply