The complement system is a group of proteins that when activated lead to target cell lysis and facilitates phagocytosis through opsonisation. Individual complement components can be quantified however this does not provide any information as to the activity of the pathway.
The total complement activity / CH50 (also known as CH100) is a screening assay for the activation of the classical complement pathway and it is sensitive to the reduction, absence and/or inactivity of any component of the pathway. The total complement activity / CH50 tests the functional capability of serum complement components of the classical pathway to lyse sheep red blood cells (SRBC) pre-coated with rabbit anti-sheep red blood cell antibody (haemolysin). When antibody-coated SRBC are incubated with test serum, the classical pathway of complement is activated and haemolysis results. If a complement component is absent, the total complement activity / CH50 level will be zero; if one or more components of the classical pathway are decreased, the CH50 will be decreased. A fixed volume of optimally sensitised SRBC is added to each serum dilution. After incubation, the mixture is centrifuged and the degree of haemolysis is quantified by measuring the absorbance of the haemoglobin released into the supernatant at 540nm. The amount of complement activity is determined by examining the capacity of various dilutions of test serum to lyse antibody coated SRBC.
You need to understand the complement pathways and the relationship of the different components in order to interpret the immunochemical results.
An increase in the plasma concentrations of C3 and C4:
--- commonly seen during inflammation. C3 and C4 are acute phase proteins and can be seen together with an increase in CRP and ESR (erthrocytesedimentation rate). As C3 and C4 increase as acute phase proteins, levels can sometimes appear normal even when the proteins are being rapidly consumed.
A decrease in both C3 and C4:
--- commonly seen when the classical pathway is activated --- immune complex-mediated diseases such as systemic lupus erythematosus / SLE --- consumption of the complement components (sepsis).
C3 is decreased and C4 is normal:
--- commonly seen when the alternative pathway is activated (Gram-negative sepsis) --- post-streptococcal glomerulonephritis --- C3 nephritic factor.
C3 is normal and C4 is decreased:
---Type II cryoglobulinaemia associated with hepatitis C infection ---C1 inhibitor deficiency ---active SLE ---genetic deficiency (C4 null alleles).
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