An unknown guinea-pig immunoglobulin with strong antibody activity to egg albumin.

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.