Figure 2. C3 deposition induced by CO antigen (1,000 ng/well) in an MBL-deficient serum that was depleted of C1qDP and Igs.

The serum was used at a final concentration of 20% and was studied with or without reconstitution of the alternative pathway. Reconstitution of undiluted serum with C1q (70 mg/l, not shown) or D (1 mg/l) together with P (25 mg/l) did not promote C3 deposition, showing that the CO antigen did not activate complement in the absence of recognition proteins. Addition of MBL/MASP complexes promoted C3 deposition in a dose-dependent manner; alternative pathway-mediated amplification was evident at moderately low MBL/MASP concentrations but was not required at high MBL/MASP concentrations. C3 deposition was measured after 60 minutes at 37°C. The experiment was performed in duplicate and was repeated once with similar results.