Shown on the right is a small, mature lymphocyte typical of those found in peripheral blood. Lymphocytes can be the most numerous cell type in certain animal species, including cattle and rodents. In dogs, cats, horses and camelids, they are outnumbered by segmented neutrophils in health. Most lymphocytes are smaller than a neutrophil, although there may normally be small numbers of intermediate to large lymphocytes, especially in cattle where up to 50% of lymphocytes can be intermediate in size. The distinguishing feature of lymphocytes is their dense, round to slightly indented nucleus with smooth (clumped or blocky chromatin), a small amount of clear to pale blue cytoplasm with a high nuclear:cytoplasmic ratio (i.e. the majority of the cell is taken up by the nucleus with only a small volume of cytoplasm visible). In most healthy animals, low numbers of lymphocytes have red cytoplasmic granules. These granular lymphocytes (see image below) are involved in cell mediated cytotoxicity and are either cytotoxic T cells (CD3+, CD8+) or natural killer cells (CD3-). Increased numbers of granular lymphocytes in blood can be seen in reactive (e.g. Ehrlichia canis infection in dogs) or neoplastic conditions (e.g. lymphoma or leukemia of granular lymphocytes). Reactive lymphocytes are cells that have increased amounts and generally deeper blue cytoplasm. Some may be larger than a normal lymphocyte but they still have clumped chromatin. Reactive lymphocytes are responding to an antigenic stimulus that is not specific. Low numbers of reactive lymphocytes are frequently seen in young naive animals. For more information (and pictures) on reactive lymphocytes, refer to the leukocyte section of the hematology atlas.
Lymphocytes, unlike the other leukocytes, are produced in lymphoid tissue rather than in 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.
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