Corticosteroids suppress the humoral antibody response of mice to sheep erythrocytes. This response depends on interactions between thymus-derived helper cells and bone marrow-derived antibody-forming cell precursors (AFC precursors). Previous experiments had shown that spleen cells (a mixture of thymus-derived and marrow-derived cells) were sensitive to corticosteroids while AFC precursors in the bone marrow were resistant. The present experiments showed that the thymus of a mouse given 2.5 mg of hydrocortisone acetate, although containing only about 5% of the number of cells of a normal thymus, was as effective as a normal thymus in cooperating with bone marrow when transferred to irradiated syngeneic mice and stimulated with SRBC. The proliferative response of thymus helper cells to SRBC was also resistant to hydrocortisone.

In this context, the majority of thymic cells are in the cortex, are rapidly dividing, are sensitive to corticosteroids and are not iminunocompetent. A small number of thymic cells, probably located in the medulla, are resistant to corticosteroids, but are immunocompetent since they can serve as helper cells.

The hydrocortisone-sensitive phase of the splenic response to SRBC was found to be the bone marrow-derived AFC precursor since spleens from hydrocortisone-treated donors had immunocompetence restored by normal bone marrow but not by normal thymus cells.

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