The complement system is the backbone of the natural immune system and was discovered over 100 years ago on the assumption that it complemented the immune cells known at the time. Since then, over 50 components of this system have been described. Depending on the trigger, a distinction is made between three different activation pathways of the complement system: the classic, the alternative and the lectin pathway.
At the end of the three pathways, the membrane attack complex (C5b-9) is formed, which can kill pathogens and cells directly. Highly effective substances are produced as “by-products”. Some mark (opsonize) particles so that they can be phagocytosed by phagocytes, others are pro-inflammatory and attract immune cells.
In rare cases, there is a defective or reduced production of factors of the complement system, which can lead to a complement deficiency and thus to an increased susceptibility to infection or to increased autoimmunity. If the cause of the complement deficiency is congenital, the affected patients often already have severe infectious diseases as children, such as meningococcal meningitis or severe pneumonia. In some diseases, such as systemic lupus erythematosus or hereditary angioedema, complement factors are consumed faster than they are produced and can therefore be an indication or measure of disease activity.
Complement determinations are therefore indicated in the following cases: in the case of immunodeficiencies, e.g. if repeated infections with encapsulated pathogens have occurred, for the detection of increased activation or insufficient regulation in the context of autoimmune diseases and nephritides. Another indication is the determination of the C1 esterase inhibitor in angioedema.