Immunological tolerance produced in rabbits by neonatal injections of BSA can be terminated by a series of injections of certain heterologous serum albumins which cross-react with BSA. Injections of albumins distantly related to BSA were more effective in terminating the tolerant state than injections of albumins closely related to BSA. It was concluded from results obtained with several heterologous albumins that immunological tolerance to BSA is directed to both the over-all antigenic or physical-chemical composition of the protein and the individual determinant groups present on the protein. Several possible mechanisms were given to explain the ability of cross-reacting albumins to terminate the tolerant state of BSA-tolerant rabbits.

A possible relationship between the termination of tolerance in BSA-tolerant rabbits injected with cross-reacting albumins and autoimmunily was discussed. It was also suggested that the relative ease with which tolerance could be established to heterologous serum proteins in comparison to bacterial antigens is the result of the close serological and physical-chemical relationship of the heterologous serum proteins to the serum proteins of the rabbit.

Rabbits injected with 500 mg of BSA during the first 5 days of life failed to form antibody capable of either eliciting an immune elimination of an injection of I* BSA given 3 to 4 months later or complexing with the circulating I* BSA.

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