Two experimental models have been used to study T-cell differentiation. The first, a graft-vs.-host reaction, was induced by injecting thymocytes or cortisone-resistant thymocytes into lethally irradiated allogeneic mice. The second was tumor graft rejection in allogeneic hosts. Ultrastructural studies at various time intervals revealed two differentiated T-cell types. One of these (the "pale" cell) is probably high cytotoxic as measured in the chromium-release assay, the other (the "dark" cell) may be an "amplifier" cell, helping in the differentiation of cytotoxic cells.

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