Near the end of the 19th century Belgian immunologist Jules Bordet discovered a heat sensitivity to antimicrobial components in blood serum. A relatively heat-stable component conferred immunological protection against specific invaders, while a heat-labile component – readily damaged by heating – provided nonspecific antimicrobial defense.
The heat-labile component was named complement by German physician Paul Ehrlich in the late 1890s. The complement system was so-named because it complements the work of phagocytes and antibodies. Antibodies are large Y-shaped proteins which act as part of the immune system.
Complements comprise a complex series of over 30 proteins found in plasma and on cell surfaces. Along with blood clotting and unclotting, complement forms one of the triggered enzyme systems in plasma.
These enzymatic systems characteristically produce a rapid, highly amplified response to a triggering stimulus mediated by a cascade, where the product of one chemical reaction is the enzymic catalyst of the next. The activated proteins or other products of the cascade serve a variety of immunological functions.
A complement kills by blasting a large hole through the membrane of an invading microbe. The loss of membrane integrity ruins a pathogen’s capability to control cell fluid equilibrium, particularly salt concentration, thus slaying it.
Host cells and commensal microbes possess protein badges that identify them as friendly, inactivating any complement cascade against them. Not surprisingly, some pathogens evolved a defense strategy that presents cascade inhibitors. But most do not have this defense.
Complement is a critical part of the innate immune system. The complement system continually operates via intricately complex chemical reactions, known as pathways.
3 human complement pathways have been identified. At least 1 pathway is shared by primitive organisms that are phylogenetically distant from humans.
Complement is instrumental playing the inflammation symphony in the innate immune system orchestra. Complement facilitates phagocytosis.
The proteins and glycoproteins comprising the complement system are synthesized in the liver.
The complement system was an early immune system development and is billions of years old. Evolution added more complement artillery to early vertebrates than what invertebrates had, and even more in mammals.
In many cases, complements recognize foreign microbes directly, without the aid of antibodies, though coordinated attack with antibodies is more efficient.