Definition:

Complement disorders account for only 2 percent of all primary immunodeficiency disorders. They result from the disruption of one of the proteins involved in the classic or nonclassic activation pathways of the complement response. Defects in the classic pathway (CP) account for the more common type of complement deficiency, and are associated with increased risk to develop systemic lupus erythematosus (SLE) and SLE-like diseases. Homozygous C2 deficiency, which is the most frequent hereditary deficiency in complement classical pathway components, is associated with SLE in 10% of the cases. Complete C1q and C4 deficiencies are less frequent but associated with a higher prevalence of SLE.