Deficiencies in any of the defense mechanism of host can lead to severe microbial infections.
Deficiencies of complement proteins are frequently associated with an immunodeficiency state similar to the condition associated with immunoglobulin deficiency.
The individuals suffer with recurrent infections from organisms that are normally susceptible to opsonisation or lysis by complement system.
A variety of hereditary and acquired complement deficiency states have been reported from early 1970s in conjunction with severe bacterial infections and, perhaps more surprisingly in individuals with disease conditions associated with autoimmunity and immune complex formation.
Generally deficiencies of complement enzymes are congenital. Properdin deficiency is X-linked, while other genetic defects within the complement system appear to be transmitted as autosomal recessive traits.
However hereditary angio-edema is due to heterozygous deficiency or dysfunction of the CI inhibitor protein. In addition, homozygous C4a and C4b deficiencies may have pathogenetic importance.
Congenital deficiency of C3 component is lethal and results in most hazardous conditions. Deficiency of C6 or C7 components may not show much effect on immunity.
Deficiencies of C2, Clr or C4 are commonly associated with autoimmune disorders such as “Systemic Lupus Erythmatosus” (SLE).
Since regulatory proteins of complement system play an important role in regulation of immunological functions, their deficiency also result in pathological conditions.
For example deficiency of CI inhibitor leads to a condition known as “hypocomplementemia” due to uncontrolled activation of the alternate pathway and the individuals with this problem suffer with hemolytic uremic syndrome.