C2 and systemic lupus erythematosus: Complement deficiencies, especially those of early components of the complement activation cascade, can lead to both recurrent infections and autoimmunity.12 Early classic complement deficiencies (C1, C2, or C4) cause susceptibility to infections with encapsulated bacteria and are frequently associated with autoimmunity, especially SLE.126,127 A deficiency of C4 or any of the components of the C1 complex results more commonly in SLE than C2 deficiency (88% of patients with C1q deficiency, 75% in the case of C4 deficiency, 57% of those with C1s/r vs. 10% in the case of C2 deficiency).