Deficiency of the 9th component of complement (C9) in a patient with systemic lupus erythematosus.

We describe the first case of systemic lupus erythematosus (SLE) showing the 9th component of complement (C9) deficiency. In the serum of this patient, a low level of serum total hemolytic complement (CH50) and the absence of C9 was demonstrated. The patient's sister, with normal level of C9, was also found to have an autoimmune disorder.