Definition:

To prevent undesirable complement activation, host tissues express a number of complement regulatory proteins (CRPs). They include C1 inhibitor (C1-INH, also termed SERPING1), C4 binding protein (C4BP), Factor I, decay accelerating factor (DAF), CD59, and so on. C1-INH (SERPING1) deficiency can be genetic or acquired, and this causes an episodic, local increase in vascular permeability in the subcutaneous and submucosal layers, identified as angioedema (hereditary or acquired). Genetic deficiency of C4BP has only been reported in one individual who developed an illness with similarities to Behcet's syndrome. Heterozygous mutations within the Factor I gene have been recently identified in individuals with atypical hemolytic uremic syndrome (aHUS), while homozygous deficiency of Factor I is an unusual feature. Factor I-deficient patients present high susceptibility to respiratory tract infections and meningitis. Kidney impairment and autoimmune disorders have also been observed. Only a single case of CD59 deficiency has been reported and this individual developed a paroxysmal nocturnal haemoglobinuria(PNH)-like illness. In contrast, deficiency of DAF is not associated with haemolytic anaemia.