Lymphocytes Feline lymphocyte Lymphocytes are the most numerous cell type in certain animal species, including cattle and rodents. Shown is a small, mature lymphocyte typical of those found in peripheral blood. Characteristic features include a dense, round nucleus and a scant rim of pale blue cytoplasm. Healthy ruminants have a wide range of lymphocytes in peripheral blood; many are quite large. In addition, in all species, there are low numbers of lymphocytes with small red cytoplasmic granules, so-called granular lymphocytes. These are either natural killer cells or cytotoxic T-lymphocytes and are involved in cell-mediated immunity. Lymphocytes, unlike the other leukocytes, are produced in lymphoid tissue and in bone marrow. Most of the lymphocytes in blood are long-lived cells that recirculate between blood and tissue. Changes in blood lymphocyte number usually reflect changes in distribution rather than changes in production or loss. Last Updated: June 2000