Metabolism of Third Complement Component (C3) in Nephritis

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Abstract
The mechanism of depression of the third complement component (C3) was studied in 20 patients with systemic lupus erythematosus and nephritis (13 with low C3), and 41 patients (21 with low C3) with chronic glomerulonephritis. The metabolism of C3 was studied by examining the plasma turnover of radioiodinated C3. The serum levels of the fourth component (C4) and properdin factor B (involved respectively in the classic and alternate pathways to C3 activation) were also examined. In patients with low serum C3, catabolic rates of C3 were elevated. Twelve of 13 patients with systemic lupus erythematosus and nephritis and low C3 had depressed levels of C4, and eight of these 12 had a decrease in serum factor B. Seven of 21 patients with chronic glomerulonephritis and low C3 had depressed serum levels of factor B, whereas serum C4 was normal in the entire group.