Bovine sera contain factors which are capable of agglutinating mouse erythrocytes and stimulating the pinocytic activity of cultivated mouse macrophages. The hemagglutinating and vesicle-inducing activities of sera increase with the age of the animal and are absent in fetal calf serum.

The majority of this material is recovered in globulin fractions prepared with Na2SO4-(NH4)2SO4 and is absent in bovine fraction II. It behaves as a macroglobulin in studies employing zone electrophoresis, Sephadex G-200 filtration, sucrose density gradient centrifugation, and in its sensitivity to 2-mercaptoethanol and heat.

Absorption of bovine sera with either mouse erythrocytes or spleen cells removes the hemagglutinating and pinosome-inducing properties of the sera.

The addition of small quantities of bovine macroglobulin to mouse macrophages results in a stimulation of pinocytic activity, phase-dense granule formation and the cellular content of three acid hydrolases.

In the presence of heat-labile factors, the macroglobulin initiates the hemolysis of mouse erythrocytes and the cytolysis of mouse macrophages.

This material is thought to represent an interspecies γM-type antibody directed against common antigenic determinants on the mouse erythrocyte and macrophage surface.

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