Studies of an Additional Kindred with Hereditary Deficiency of the Second Component of Human Complement (C2) and Description of a New Method for the Quantitation of C2

Abstract

The present paper reports studies of a third kindred which manifests the hereditary C2 deficiency trait. The homozygous individual had no detectable hemolytic serum complement or C2 activity, although C2 could be detected by immune adherence; three heterozygous members of the family had 60 to 70% of the normal serum C2 titers. Normal or elevated serum C1, C3, C4 and C5 levels were obtained in all members of the kindred. There was no evidence for a C2 inhibitor in the propositus' serum; in addition, the deficiency could be corrected by the addition of purified C2. An analytical technique which permits quantitation of very small amounts of C2 was developed and applied to the measurement of C2 in the serum of members of the family. It is based upon the quantitation of C3 uptake by EAC1, 4 as a function of C2 concentration, i.e., the immediate reaction product of a C2-dependent reaction is measured. The serum C2 level was found to be 0.5 to 4% of normal in the homozygous individual and 60 to 70% of normal in the heterozygotes. When normal sera were assayed by this technique, the C2 titers obtained were in good agreement with the values obtained by hemolytic titrations.