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A New Serum Factor in Normal Rabbits¹

II. Reaction with Buried Antigenic Determinants Revealed after Papain Digestion

From the Department of Microbiology, The University of Texas, Austin, Texas

Abstract

Homoreactant, present in the purified normal rabbit G-globulin, reacted specifically with homologous Fab fragments. The reaction was demonstrated by the agglutination of human red cells sensitized with the Fab of rabbit anti-HEA. Pepsin digestion of HR did not affect this agglutinating activity; it was destroyed, however, by papain digestion. The activity of HR was inhibited by the Fab fragments of normal rabbit G-globulin. The concept that HR is a 7S naturally occurring antibody is supported by the data.

Homoreactant was unable to agglutinate erythrocytes sensitized with 1) the artificially prepared univalent 7S hybrid anti-HEA, 2) subagglutinating doses of intact anti-HEA or 3) the rabbit incomplete antierythrocyte antibody. Furthermore, HR was not inhibited by intact G-globulin. The data suggest that HR is directed to antigenic determinants which are ordinarily buried in the intact G-globulin molecule and are exposed after papain digestion.

Gel-diffusion experiments provided another way of demonstrating HR activity. The data suggest that this technique may prove to be useful for further quantitative studies.

Differences between HR and certain other serum factors are discussed.

Footnotes

¹ Supported by the United States Public Health Service Grant AI-07184-01.