Serum from rabbits rendered tolerant or immune to 100 MPD-3/kg of endotoxin when passively transferred to normal rabbits gave partial tolerance to the standard dose of endotoxin. The same serum was fractionated by DEAE chromatography into 4 major fractions. Immunoelectrophoretic analysis indicated that the 7S γ2-and the 19S γ1-immunoglobulins were separated into two distinct fractions. Of the four fractions tested, only fraction IV containing 19S γ1-immunoglobulm conferred complete pyrogenic tolerance to 100 MPD-3/kg of endotoxin.

Additional fractionation of DEAE fraction IV by exclusion chromatography on sephadex G-200 gave 3 fractions. Of these only the first, containing 19S γ1-immunoglobulin conferred complete pyrogenic and lethal tolerance to normal rabbits. There was no correlation between the quantity of O-specific antibodies and the ability to transfer tolerance.

It is concluded that endotoxin tolerance involves a classical immune mechanism which includes both 19S γ1-immunoglobulin specific for toxophore groups common to many endotoxins and a normally functioning RES.

To avoid confusion with immunologic tolerance, it is suggested that the term endotoxin immunity be substituted for endotoxin tolerance.

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