Peripheral blood lymphocyte subpopulations and immunoglobulin concentrations in healthy foals and foals with Rhodococcus equi pneumonia

Infectious diseases are common in foals aged 1-5 months. The objectives of this investigation were to evaluate immunologic parameters in foals from bi rth to weaning to establish reference values for the proportion of circulat ing lymphocytes that were helper (CD4+) or cytotoxic (CD8+) T cells, or B c ells; to measure serum immunoglobulin (IgM and IgG) concentrations; and to compare these immunologic parameters to values in foals with naturally occu rring Rhodococcus equi pneumonia and in adult horses. Peripheral blood lymp hocyte subpopulations were determined by how cytometric analysis, and serum IgG and IgM concentrations were determined by radial immunodiffusion. Flow cytometric analysis of lymphocyte subpopulations suggested age-related cha nges in the cell-mediated immune system in horses. Absolute circulating CD4 + and CD8+ T lymphocytes and B cells increased linearly up to 3 months of a ge. Circulating B cell concentrations from birth to 6 months of age were gr eater than values in adult horses and the lymphocyte differences among the age groups are mainly due to variation in B lymphocytes. Both absolute and proportional B cell concentrations were greater in foals with R equi pneumo nia than in healthy foals at the same age. The increase in absolute cell co unts of each subpopulation was dependent on the increase of absolute periph eral blood lymphocyte count. Serum IgG concentration increased linearly fro m 1 to 3 months of age, and serum IgM concentrations increased from 1 to 6 months of age. These data suggest age-dependent cell-mediated and humoral d evelopment in young foals.