Abstract
Complement system aberrations have been identified as pathophysiological mechanisms in a number of diseases and pathological conditions either directly or indirectly. Examples of such conditions include infections, inflammation, autoimmune disease, as well as allogeneic and xenogenic transplantation. Both prospective and retrospective studies have demonstrated significant complement-related differences between patient groups and controls. However, due to the low degree of specificity and sensitivity of some of the assays used, it is not always possible to make predictions regarding the complement status of individual patients. Today, there are three main indications for determination of a patient's complement status: (1) complement deficiencies (acquired or inherited); (2) disorders with aberrant complement activation; and (3) C1 inhibitor deficiencies (acquired or inherited). An additional indication is to monitor patients on complement-regulating drugs, an indication which may be expected to increase in the near future since there is now a number of such drugs either under development, already in clinical trials or in clinical use. Available techniques to study complement include quantification of: (1) individual components; (2) activation products, (3) function, and (4) autoantibodies to complement proteins. In this review, we summarize the appropriate indications, techniques, and interpretations of basic serological complement analyses, exemplified by a number of clinical disorders.
Highlights
Reviewed by: Marco Cicardi, Università degli Studi di Milano, Italy Peter F
We summarize the appropriate indications, techniques, and interpretations of basic serological complement analyses, exemplified by a number of clinical disorders
The complement system comprises approximately 50 proteins that are found in the fluid phase of the blood or bound to cells where they function as receptors or regulators of complement activation (Figure 1A)
Summary
Complement system activation via different pathways in blood plasma is a feature of a large number of diseases. Functional tests, in particular different haemolytic assays that monitor a whole activation pathway from the recognition phase to MAC-formation (=lysis) can be used to detect both deficiencies in individual component as well as depression in complement function caused by consumption of intact complement components. Since most plasma complement components are commercially available, it is possible to verify the deficiency by reconstituting the patient sample with the protein in question and repeat the functional assay, where the activity should be normalized. Complement analyses, in particular determination of CP function and analysis of components within the CP: C1q, C3, and C4 (C2 in some laboratories) are useful markers to monitor disease activity and for differential diagnosis (Figure 7). Western blot analysis of the separated C1q A, B, and C chains (after reduction) has been used to show that anti-C1q antibodies have different binding specificities in SLE than in (HUVS) [42, 59]
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