Systemic lupus erythematosus (SLE) syndrome is often found associated with deficiency in C1q, C1r and C1s (Table 1) . SLE in humans is characterized by fever, rash, glomerulonephritis and, sometimes, hemolytic anemia. SLE is associated with the presence of anti-nuclear antibodies and extractable nuclear antigen (ENA) and absence of anti-DNA antibodies. It is thought that absence of key complement proteins results in: (1) in defective immune complex clearance and consequent deposition of the complexes in various organs, especially in the kidney and in arterial walls (causing vasculitis); (2) defective recognition of self by B cells leading to autoimmunity; and (3) defective disposal of dying cells including B and T cells, again leading to autoimmunity. C2 and C4 deficiency is also associated with development of SLE, although the correlation is not as strong as with C1q deficiency. C3 deficiency is very rare in humans and does not usually lead to SLE. However, deficiency in C2, C3 and C4 increases the susceptibility to infections by bacteria Haemophilus influenzae and Neisseria (for a comprehensive review on complement deficiencies and resultant clinical manifestations, see Botto et al. 2009; Pettigrew et al. 2009 ).