The thymus is essential to the development of cell-mediated hypersensitivity as well as certain types of antibody formation (1–3). An interaction of thymus-derived and bone marrow-derived cells apparently is required for many of these responses (4–8). In cases studied thus far which involve hemolysin production against sheep erythrocyte antigens, thymus-derived cells show an initial mitotic response to the antigenic stimulus, but the actual antibody-forming cells have proved to be marrow-derived (5–8). In tuberculin hypersensitivity, which may be taken as a well-established example of cellular hypersensitivity, while the thymus is required for active sensitization the overwhelming majority of cells which leave the circulation and infiltrate specific delayed skin reaction sites are nonspecific marrow-derived monocytes (9–11). The data presented here establish the presence of significant proportions of thymus-derived cells in the early stages of the tuberculin reaction in the rat.

The animals, materials and techniques used in this study have been described in detail elsewhere (10, 12).

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