The complement system has important protective functions in both, the innate and the adoptive immune systems. Dysregulation of complement system can cause the tissue damage. The complement deficiency predispose to an infection, and also to a development of auto-immune diseases, especially systemic lupus erythematosus (SLE). The complement play an important role in the pathogenesis of SLE, and besides that the lower levels of complement's compounds are well recognised as marker of SLE activity and also the activity of secondary APS.