Complement Immunossay Products
The complement system is a complex system of proteins that acts as a cascade. Each protein within the system is assigned a number and they react in sequence once the system has been activated. Many of the proteins are pro-enzymes that require proteolytic cleavage in order to become active. The complement cascade forms part of the body’s innate immune system and is involved in host defense against infection, the initiation of an inflammatory response and the destruction of certain bacteria and viruses.
The complement cascade can be activated in 3 different ways, all of which lead to the formation of a convertase that cleaves C3 to form C3a and C3b. C3b in turn activates C5 and the remainder of the cascade which leads to the formation of the Membrane Attack Complex.
Total deficiency of a complement protein is rare. The absence or reduction of a complement component usually results from consumption due to activation. This can be caused by an autoimmune disease or frequent infection. The prevalence of complement deficiency related to PID has been reported as 2.5% (from data collected by 4 major PID registers).1 Higher frequencies have been reported in other countries. The measurement of complement CH50 activity is now recommended as part of the diagnostic protocol for PID.2,3 It can also provide important information for many other disease states such as Systemic Lupus Erythematosus (SLE) and bacterial infections.4
Screening
The CH50 method is used as a screening test to detect a deficiency in the classical and terminal complement pathways. It measures the lysis of antibody-sensitized sheep erythrocytes after addition of fresh serum. The result is expressed as the reciprocal of the dilution that yields 50% erythrocyte (red blood cell) lysis. In comparison the CH100, or total hemolytic complement, is a measurement of 100% lysis of the red blood cells. The AH50 (or AH100) is used to detect a deficiency of the alternative complement pathway.
Quantitative
If a deficiency in one of the complement pathways is detected after screening, the level of individual complement components may be measured separately. Deficiencies of specific complement components are related to a wide variety of disease states; for example, C3 deficiency is associated with a high susceptibility to bacterial infections. C3 deficient patients may present with similar symptoms to patients with severe antibody deficiency or defects of phagocyte function.3
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- Rezaei N, et al. An introduction to primary immunodeficiency diseases. Primary immunodeficiency diseases 2008; 5:1-38
- De Vries, E. Patient-centred screening for primary immunodeficiency: a multi-stage diagnostic protocol designed for non-immunologists. Clin Exp Immunol 2006; 145:204-214
- Bonilla FA, et al. Practice parameter for the diagnosis and management of primary immunodeficiency. Ann Allergy Asthma Immunol 2005; 94:S1-63
- Botto M, et al. Complement in human diseases: Lessons from complement deficiencies. Mol Immunol 2009; 46:2774-2783
