"Natural" antibodies to representative species of Enterobacteriaceae were found to be principally associated with the γ1-macroglobulin prepared from Cohn fraction III-I of pooled human plasma. The amount of antibodies present was estimated by measuring the bactericidal, hemagglutinating, mouse-protective, and opsonophagocytic activities of this fraction. Comparison with the antibody activity of γ2-globulin of Cohn fraction II showed that up to 100 times more antibody activity on a weight basis was present in the macroglobulin fraction. These findings suggest that differences between natural and immune antibody result from the physicochemical properties of 2 different classes of immune globulin.

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