Distribution of Immunocompetent Cells in Immuno-Organs of Horses

Abstract

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.