Automated hematology analyzers are routinely used to evaluate lymphocytes and detect changes in concentration (e.g., lymphopenia, lymphocytosis). However, hematology analyzers cannot reliably detect morphologic changes or classify cells (e.g., large lymphocytes). Therefore, manual blood smear review is critical for accurate lymphocyte assessment. This article describes peripheral lymphocyte physiology, variations in peripheral lymphocyte morphology, and conditions that commonly cause variations in lymphocyte concentration.<\/p>\n
Take-Home Points<\/strong><\/p>\n <\/div><\/div><\/p>\n A CBC is an integral part of a minimum database that identifies changes in cellular concentrations and morphologies, which can help diagnose disease and illness. Automated hematology analyzers are helpful for determining cellular concentrations. However, they cannot reliably assess morphologic changes or classify cells.1<\/sup> Therefore, blood smear review is recommended to evaluate cellular morphologies and confirm analyzer quantifications. This article focuses on lymphocyte abnormalities, primarily morphologic and concentration changes.<\/span><\/p>\n A blood smear review should be performed if lymphocytosis is reported. A few examples of additional analyzer flags that should prompt a blood smear review include monocytosis, suspected blast cells (especially if lymphocytes are more immature or large), and abnormal leukocyte distribution. As previously described in the January\/February 2025 issue (<\/span>go.navc.com\/4fxiLVu<\/span><\/a>), systematic review of a properly prepared and stained blood smear is recommended.1<\/sup> Because cellular distortions may occur in the feathered edge and body of the smear, lymphocytes should be identified in the monolayer. However, if large lymphocytes are present in low numbers, they may occur predominantly in other parts of the blood smear like the body. Thus, it is recommended to screen the entire smear.<\/span><\/p>\n Lymphocytes are the primary mononuclear cells in peripheral blood, and they are responsible for several immune functions. Lymphocytes are derived from bone marrow stem cells that mature into T or B cells.<\/span><\/p>\n T cells develop in the thymus, then circulate or accumulate in lymphoid tissues, such as lymph nodes and the spleen. T cells are the primary lymphocytes found in the blood of healthy dogs and cats. They are responsible for cell-mediated immunity and include helper T cells (CD4+<\/sup>) and cytotoxic T cells (CD8+<\/sup>).<\/span><\/p>\n Depending on the species, B cells develop in the bone marrow or specialized lymphoid tissue (e.g., ileal Peyer\u2019s patches, bursa of Fabricius). B cells are responsible for humoral immunity via production of immunoglobulins, also known as antibodies. There is also a population of null cells that do not express T or B cell markers, which includes natural killer (NK) cells, which play essential roles in the innate immune system, particularly defending against viral infections. The different subsets of lymphocytes cannot be reliably distinguished by light microscopy, yet flow cytometry<\/a> can identify different cellular markers and receptors (immunophenotyping).2<\/sup><\/span><\/p>\n When morphologic characteristics of lymphocytes are evaluated, cell size as well as nuclear and cytoplasmic qualities should be noted (<\/span>BOX 1<\/b><\/span>). Normal lymphocytes in healthy dogs and cats are small\u2014a distinction made by comparison to a neutrophil. A normal lymphocyte is smaller than a neutrophil; an intermediate lymphocyte is approximately the same size as a neutrophil; a large lymphocyte has a bigger diameter than a neutrophil.<\/span><\/p>\n Vacuoles <\/strong><\/p>\n Inclusions <\/strong><\/p>\n\n
Lymphocyte Populations<\/h2>\n
Lymphocyte Morphology Variations<\/h2>\n
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