ACQUIRED immunological tolerance of tissue grafts has been induced in adult mice of several strains either by single intravenous injection of large numbers of viable lympho-reticular cells or by repeated administration of large numbers of these cells intravenously and intraperitoneally over a prolonged period1–3. The number of cells injected as well as the length of time during which treatment must be maintained to achieve tolerance appeared to vary with the degree of difference in histocompatibility between the host and the donor involved. For example, while tolerance of male skin isografts was obtained in adult female mice of the C57BL/1 and A strains by a single intravenous injection of 20–30 million spleen cells taken from male donors, a total of 1,500 million viable homologous cells administered over a period of 7 weeks was necessary to induce tolerance in C3H/Bi mice to (A × C3H)F1 hybrid skin homografts. In the former instance the histocompatibility difference between male and female mice is considered to be ‘weak’4 while, in the latter, donor and host differed at the ‘stronger’ H-2 histocompatibility genetic locus.
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