Selective deficiency of c1s associated with a systemic lupus erythematosus—like syndrome. Report of a case

We describe a patient who developed a systemic lupus erythematosus‐like syndrome characterized by bilateral malar erythema, antinuclear antibody, and anti—double‐stranded DNA antibody. He was started on hemodialysis (3 times/week) because of renal failure. He completely lacked total hemolytic complement (CH50) activity, which was subsequently determined to be due to the absence of the first component of complement (CI). The specificity was further defined, by Ouchterlony analysis using anti‐CIs antiserum, and was found to be the C1 subcomponent C1s. There was no absence of C1r. We conclude that this is a case of selective deficiency of C1s.