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Evidence is provided which supports the concept that a previously characterized monoclonal IgA-λ rheumatoid factor with a medium-binding avidity reacts with a select Fc region antigenic determinant. The interaction between this rheumatoid factor and anti-Rh-coated human erythrocytes is inhibited by IgG1 Fc fragment, IgG1 heavy chains, and a single peptide pool isolated from this tryptic digest by paper chromatography. Cyanogen bromide cleavage of the isolated heavy chains abolishes this reactivity. A methionine residue is assumed to be near, involved in, or necessary for reactivity between the IgA and its antigenic determinant. The data thus provide further evidence for a specific autoantigenic, rheumatoid factor-reactive determinant. [ABSTRACT FROM AUTHOR]

In this paper the specificity of the skin-reactive guinea-pig immunoglobulin in the homologous PCA test has been reconsidered. Extensive purification of this immunoglobulin by chromatographic methods results in the isolation of a small fraction or medium speed immunoglobulins that contains all skin reactivity. No correlation could be found with the 7S immunoglobulins IgGγ1 and Igγ1A suggesting a separate class of biologically active immunoglobulins. The PCA active antibody belongs to either an IgG subclass or is a heat stable homocytotropic antibody. Rabbit and chicken antisera against isolated fractions of guinea-pig immunoglobulins show the existence of 7S subclasses. Some suggestions for a revised nomenclature are given. [ABSTRACT FROM AUTHOR]