The complement system consists of many plasma proteins which are produced by hepatocytes and take part in innate and adaptive immunity.
When this complement system undergoes proteolytic activation , it propagates an enzymatic cascade which further leads to recruitment
of inflammatory cells, amplificationof their phagocytic property and formation of membrane complexesthat promote destruction of microbes.
The activation of the complement system is by two reaction pathways which are alternative and classical. Alternativepathway is triggered
by microorganisms and the classical pathsay by cell bound immune complexes.
The complement pathway consists of recognition mechanism lead by CLq, CLr and CLs, activation mechanism by C4, C2 and C3 and finally
the attack mechanism by C5, C6, C7, C8 and C9. Activation of C3 involves its division into C3a and C3b byenzyme C3 convertase.
Many recurrent bacterial infections are know to occur when there is a deficiency of C3.
C4 also is one of the activation proteins taking part in the classical pathway. When this protein is abset,
the C3 activation pepticles are unable to clear the immune complexes formeddue to bacterial infections. Decreased
levels of C4 are generally associated with lupus and rheumatoid arthritis.
