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About 50 per cent of guinea-pigs immunized with egg albumin possess strong antibody activity in an immunoglobulin class with antigenic properties different from those of the well-known guinea-pig immunoglobulins. This immunoglobulin is present in normal guinea-pig serum in very small amounts and its concentration rises during immunization with egg albumin. Immunization with bovine insulin or bovine γ-globulin does not stimulate increased synthesis of this immunoglobulin class. Owing to the special conditions that are needed for its stimulation it is provisionally called IgS. It was found to be a heat stable 6·5S immunoglobulin and is not involved in PCA reactions. [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]