Complement- Properties and Nomenclature


The complement system is a vital part of the immune system that helps to eliminate infectious microorganisms and damaged cells from the body. It also enhances the functions of antibodies and phagocytes, which are other immune cells that recognize and destroy foreign invaders. The complement system consists of a complex network of proteins that circulate in the blood and tissues, and are activated by various triggers, such as bacteria, viruses, or antibody-antigen complexes.

The complement system has several important roles in immunity, such as:

  • Causing the lysis (bursting) of cells that are infected or harmful to the body, such as bacteria, viruses, tumor cells, or transplanted organs.
  • Generating mediators that stimulate inflammation, which is a protective response that attracts more immune cells to the site of infection or injury.
  • Facilitating opsonization, which is a process that coats the surface of foreign particles with molecules that make them easier to be engulfed and eliminated by phagocytes.
  • Causing immune clearance, which is a process that removes immune complexes (aggregates of antigens and antibodies) from the circulation and transports them to the spleen and liver for disposal.

The complement system is named so because it "complements" or enhances the ability of antibodies and phagocytes to clear microbes and damaged cells from the body. It is part of the innate immune system, which is the first line of defense against infections and does not change or adapt during an individual`s lifetime. However, the complement system can also be recruited and activated by antibodies generated by the adaptive immune system, which is the second line of defense that can recognize specific antigens and produce memory cells for future protection.

The complement system was discovered in the late 19th century by several scientists who observed that serum (the liquid part of blood) had antimicrobial activity that was lost when heated. They also found that this activity was restored when heated serum was mixed with fresh serum from an immunized animal. They called this heat-sensitive component "complement" and the heat-stable component "antibody". Later, it was found that complement consists of many different proteins that act in a cascade-like manner to produce various effects on target cells and molecules.

In this article, we will discuss the properties and nomenclature of the complement system, as well as its main effects on immunity.