What is the complement?

The serum complement is a set of small proteins called C1, C2, C3, C4... (C for complement) present in the blood and tissues. Its role is to regulate the immune system by facilitating the elimination of microbes and cell debris, for example destroyed by a virus. The complement is in a way the "garbage collector" of the immune system. Thus, a complement defect leads to a "clutter" of debris. The cluttering of an immune system, particularly subject to stimulus (autoimmune), facilitates the appearance of an autoimmune reaction against debris of our own cells, which amplifies the autoimmune reaction.

Why is the complement measured in lupus?

The interest is twofold:

In some lupus, for genetic reasons, there is less production of certain fractions of the complement (especially C4, rarely C1 or C2).

Most often, in practice, routine tests measure the C3 and C4 fractions, but also the total complement, called CH50 (total hemolytic complement). This dosage makes it possible to monitor the activity of lupus, because in case of abnormal activation of immunity, the complement proteins (C3 and C4 particularly) rate have been decreased in the blood.

Thus, a " hypocomplementemia" by consumption generally reflects an active period of the disease, quite often with kidney damage.

However, this hypocomplementemia is not constant, especially when there is no kidney damage. The measurement of the complement is used routinely to monitor lupus.