A study on dogs with spontaneous acute distemper was carried to investigate the alteration of T and B cells subsets relating to immunosuppression and lymphocytic depletion in lymphoid tissues. CD3, CD4, CD8 and CD45RO antigen expression of T cells and IgG and IgM antibody expression of B cells and distribution of canine distemper virus (CDV) antigen were detected in lymphoid tissues and bone marrow by immunohistochemistry. The results showed the CDV antigen presence in lymphocytes, follicular dendritic cells, interdigitating cells, macrophages and myelocytes. Compared to control group, a moderate loss of CD3+and CD8+ cells (P<0.05), significant loss of CD4+ and CD45RO+ cells (P<0.01) were found in T cell areas of lymphoid tissues. The ratio of CD4+/CD8+ cells was about 0.8:1. There were lots of IgG+ cells with a few  scattered CD3+,CD4+and CD8+cells, few IgM+ and CD45RO+ cells in the follicle-like areas. Many IgG+ cells were detected in B cell areas, but the total of IgG+ cells were sharply reduced. The decrease orders of lymphocytic subsets were distinct in IgG+ cells and CD4+ cells, moderate in IgM+ cells and CD45RO+ cells, mild in CD8+ cells and CD3+ cells. The main reason of immunosuppression is the significant decrease of CD4 cells lead to the lower CD4/CD8 ratio, and low IgG+ cells lead to lower concentration of antibody against CDV. The reduce amount of lymphocytes especially B cells in lymphoid tissues lead to lymphoid depletion and the regeneration of immunocytes is evidently reduces in bone marrow.

Key words: B cells, Canine distemper virus, Lymphoid tissue, Subsets, T cells