SummaryThe complement system is activated via two pathways: the classical pathway which comprises C1, C4 and C2 activation and the alternate pathway in which properdin. C3 and factors B and D are implicated. Complement activation by the classical pathway is induced by Ag-Ab complexes whereas the alternate pathway can be activated by other substances such as polysaccharides. Both pathways lead to the activation of C3 and of the last complement components (C5 to C9).Congenital complement deficiencies affect mainly C1 esterase inhibitors, Clq, Clr, C2, C4, C3, C3b inactivator, C5, C6 and C7. Some of these deficiencies are very well supported by the patient whereas others generate a higher susceptibility to infections, glomerulonephritis and lupus syndromes.Acquired deficiencies of the complement system are always the consequence of other pathological phenomena. Most often they are characterized by a low level of the complement components which are decreased during an activation of the complement system via the classical pathway. In all probability, they are the result of an activation of the complement system by Ag-Ab complexes.Activation of the complement system can be systemic as in SLE, or localized in some extra-vascular fluids such as synovial fluid in the joints of patients suffering from rheumatoid arthritis.In some kidney diseases, the complement profile suggests that there is an activation of the complement system via the alternate pathway.