Erythema nodosum leprosum (ENL) is often interpreted as a manifestation of immune-complex deposition in patients with lepromatous leprosy. We used a C1q precipitin assay technique to demonstrate directly the presence of immune complexes in sera from patients with leprosy. Seven of 15 patients with ENL had serum C1q precipitin activity; serial tests often showed continued C1q precipitin activity. Only 3 of 27 lepromatous patients without ENL had positive tests; multiple positive tests were not seen in these patients. Single positive tests were encountered in patients with borderline (dimorphous) leprosy, especially those with downgrading reactions. There was no relationship between C1q precipitin activity and serum levels of C3, CH50, or cryoglobulins. Single positive C1q precipitin tests may occur sporadically in patients with leprosy, but repeatedly positive tests are closely associated with occurrence of ENL, supporting the concept that ENL is a complication related to the presence of circulating immune complexes.
Dr. Gelber was a Fellow in Infectious Diseases (Eli Lilly & Co.).