Abstract
SYNOPSIS. A state of immunological tolerance, or specific unresponsiveness to foreign antigen, can be induced in amphibians by joining two nonsibling embryos in parabiotic union. The induced tolerance is immunologically specific; the co-partners are tolerant of each other' santigens but do respond to unrelated antigens from third parties. The parabionts are blood-cell chimeras; each contains blood elements of the type proper to the other. The degree of chimerism wanes in the later life of the parabionts, which may be the consequence of the appearance of a new clone of uninhibited reactive cells derived from the bone marrow. Tolerance to a particular antigen is interpreted as arising by the deletion of the population of competent lymphocytes capable of responding to that antigen. The breakdown of tolerance occurs through the emergence of a new population of antigen-reactive cells. In essence, a tolerant cell as a discrete entity does not exist.
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