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Antigenic Properties of the Fab Fragment of Human IgM: Antigenic Determinants Common to IgM and IgG¹

From the Division of Human Genetics, Department of Human Biological Chemistry and Genetics, The University of Texas Medical Branch, Galveston, Texas 77550 and the Department of Medicine, State University of New York School of Medicine, Buffalo, New York 14215

Abstract

An antigenic determinant common to human IgM and IgG was revealed by the use of a rabbit antiserum produced against the Fabµ fragment of a monoclonal IgM ( type). This antiserum (R8), after absorption with Bence Jones protein, still cross-reacted with IgM and IgG. Sheep erythrocytes coated with either IgM or IgG of type were tested with 13 Waldenström IgM, 12 myeloma IgG and 3 myeloma IgA proteins by hemagglutination inhibition. Three out of five IgM () and two out of seven IgG () were markedly inhibitory at concentrations of 0.2 to 4 µg/ml.

Mild reductive cleavage of IgM did not alter its reactivity. The inhibition of Fabµ fragments appeared to parallel that of the parent IgM molecules. Isolated heavy and light chains from macroglobulins produced negligible reactivity. These studies suggest that the conformational antigenic determinant(s) of the anti-Fabµ serum may involve primarily the interaction of the Fd fragment and chains of either IgM or IgG molecules. In addition, by using hemagglutination inhibition technique, other antibody system(s) have been found in R8 which show specificity for the Fabµ fragments of both and macroglobulins.

Footnotes

¹ This work was supported by Grant H-380 from the Robert A. Welch Foundation and United States Public Health Service Research Grant 1SO1 FR-05427-09. Presented in part at the 23rd Meeting of the American Society of Human Genetics, Indianapolis, Indiana, 1970.

² Address correspondence to Dr. J. P. Chen, Division of Human Genetics, Department of Human Biological Chemistry and Genetics, The University of Texas Medical Branch, Galveston, Texas 77550.