Complement components in three pathological sera: relation to clinical states.

Sera from three patients with systemic lupus erythematosus, hereditary angioneurotic oedema and erythema exudativum multiforme, respectively, have been investigated for total complement (C'), complement components (C'1, C'2, C'3, C'4), including β1C-globulin, as well as inhibitory and enzymatic substances which are capable of inactivating C' components. All three sera had a low total C' activity. The serum from the patient with systemic lupus erythematosus had low levels of C'4, C'2 and β1C-globulin, and the reduction of total C' could possibly be attributed to an antigen–antibody reaction in vivo. It is known that the low total C' activity and the pronounced lowering of C'1, C'4 and C'2 in serum from patients with hereditary angioneurotic oedema is due to the enzymatic inactivation of C'4 and C'2 by C'1 esterase. This mechanism has been confirmed in this investigation. Serum from the patient with erythema exudativum multiforme showed only slightly reduced C'1, C'2 and C'3 activities, whereas there was a pronounced reduction of β1C-globulin and of C'4 activity as determined by the classical titration technique. However, C'4 activity was normal when determined with a stepwise technique. As the patient recovered, the C' components returned to normal levels but the C'4 titre increased only moderately. Inhibitory or enzymatic substances could not be found in the erythema multiforme serum. The low total C' activity in the erythema multiforme serum, therefore, is unlike that found in the systemic lupus erythematosus serum or the hereditary angioneurotic oedema serum. Obviously, different mechanisms account for the low total C' activities in the sera from these patients.