The complement system is an essential component of the human immune system, enhancing immune response. The proteins that are part of the complement system, .i.e. the complement factors, destroy intruding pathogens such as bacteria (known as antigens). They adhere to antigens to mark them and cause other components of the immune system to ingest and dissolve them ( phagocytosis). The complement system also triggers inflammatory responses that reinforce the immune reaction.

The German chemist and medical doctor Paul Ehrlich (1854-1915) coined the term "complement", which he understood to be a component of the immune system that complements the immune reaction. Belgian biologist Jules Bordet (1870-1961) found out what the complement system really does, i.e. he found that it enhances the antigen- antibody reaction, resulting in a more effective defence against pathogens.

More than 30 proteins make up the complement system. In their inactive form, they are found in the blood plasma. They are called complement factors and carry both the letter C and a number for identification – e.g. C1, C2, C3 and C4 – or they are called factor, followed by a letter, e.g. factor H or factor I. Once the reaction chain of the complement system has been set in motion, the complement factors react with each other through cleavage and recombination. The cleaved complement factors are assigned small letters. Thus, C3a and C3b are cleavage products of C3. Recombined complement factors or complexes are given a name made up of their components, such as C4bC2a complex.

These reactions produce a cascade of complement factors. A single activated complement factor may be enough to trigger a multiple immune response. Three different pathways for the activation of the complement system can be distinguished:

• the classical pathway
• the lectin pathway
• the alternative pathway

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Article Index
1 The Complement System
2 Classical activation pathway