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Activation of the Alternate Complement Pathway by Human Immunoglobulins

National Institutes of Health, Bethesda, Md. 20014

Abstract

A series of human myeloma proteins were prepared in highly purified form by standard chromatographic techniques. Aliquots were aggregated with bisdiazo-benzidine (BDB) over a wide range of protein and BDB concentrations. The aggregated proteins were tested for their ability to activate the alternate complement pathway in C4-deficient guinea pig serum, as evidenced by their ability to lower the titer of C3-9 after incubation in serum for 1 hr at 37°C. The number of proteins which led to C3-9 fixation/total number of proteins tested was as follows: IgG₁, 2/2; IgG₂, 1/1; IgG₃, 1/1; IgG₄, 3/5; IgM, 2/2; IgE, 1/1; IgA, 2/7. Incubation of C4-deficient serum with IgG₁ led to most complement fixation. IgG₄ was least active on a milligram basis. Both a and b subgroups of IgG₄ and both ₁ and ₂ subgroups of IgA fixed C3-9 in individual cases. In no instance did the unaggregated protein lower the titer of C3-9.