Normal and irradiated rabbits were given small doses of I131 bovine gamma globulin (BGG) and the rates of its loss from their blood were determined. The figures agreed with those of previous reports.

3 to 4 months later, both groups were reinjected with antigen. The control group gave an accelerated (anamnestic) rate of loss, indicative of the immune state. The irradiated group gave a response similar to that given by the control group on primary injection.

Rabbits recover from 500 roentgens of x-ray in less than 2 months. Hence, rabbits given BGG soon after irradiation should become immune to this antigen in 2 months if they retain it. Since rabbits reinjected with BGG 3 to 4 months after irradiation did not give an accelerated response, the BGG must not have been retained over the period of time necessary for recovery; i.e., less than 2 months. As the rate of antigen loss is greater in normal than in irradiated animals, normal rabbits will have lost BGG as active antigen prior to the irradiated animals.

The amounts of antigen used (0.25 to 0.4 mg./kg.) more nearly approximate the amounts present during disease than have the amounts used in previous studies of antigen retention.

The hypothesis that protein antigens are lost as fast or faster than homologous proteins is discussed and the conclusion reached that this is a valid concept.

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