From 15 days before to 60 days after metamorphosis, an immunologically specific tolerance to skin allografts between individuals of Xenopus can be induced by skin allografts. Genetic inheritance of histocompatibility antigens determines the degree of tolerance during metamorphosis: in individuals from the first generation, 25-50 % of the siblings were tolerant. In second generation individuals, the percentage of tolerance is 75-100 % when the parents show mutual tolerance to skin allografts at metamorphosis and only 25 % when the parents are mutually incompatible. In view of the characteristics of this tolerance it is suggested that it concerns mainly weak histocompatibility antigens and that it occurs preferentially at metamorphosis, as at this stage the animal becomes tolerant to many new self-antigens.

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A total of 694 sera have been tested in a screening program aimed a t identifying monospecific reagents for HLA-DR typing. All sera were first tested on a panel of enriched B and T cells without absorption on platelets. Only sera reacting more frequently on B than on T lymphocytes were absorbed on platelets and retested on the panel. This procedure saved a considerable amount of platelets and proved to be reasonably efficient. One-hundred-and-fifty sera were found to contain an anti-B cell antibody. Significantly less frequent B cell reactivity was found when HLA-A, -B, -C antibodies could not be detected. Twenty-five sera were demonstrated to be specific for well-defined DR antigens.

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