The distribution of immunocompetent cells (CD4, CD5, CD8, and IgM positive cells) in the immuno-organs of healthy nulliparous mares was investigated by the immunohistological technique with monoclonal antibodies. Aggregated CD4
+ or CD8
+ T cells were observed in the thymic cortex, and a small number of CD4
+ or CD8
+ T cells were found in the thymic medulla. CD5
+ T cells were observed widely in the thymus, they were detected at higher frequencies in the medulla than the cortex, but, a small number of IgM
+ B cells were scattered only in the thymic medulla. T cell-dependent areas of the spleen, lymph nodes and ileum were identified by CD5
+ T cells. In T cell-dependent areas of these organs, the number of CD4
+ T cells was slightly greater than that of CD8
+ T cells. In the center of the follicles of these organs, IgM
+ B cells aggregated, and these follicles adjoined the T cell-dependent areas. These findings suggested that T and B cells differentiate and mature in thymic and peripheral lymphoid tissues, respectively, and also suggested that immunological actions of peripheral lymphoid tissues would be related to humoral immune responses.
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