| The white blood cells consist of the granulocytes (neutrophils in most mammals, heterophils in rabbits, reptiles, and birds; eosinophils; and basophils) and the agranulocytes (lymphocytes and monocytes). Although they are traditionally counted by determining each as a percentage of the total WBC population, meaningful interpretation requires that the absolute number of each type be calculated by
multiplying the total white cell count by the fraction attributable to the individual cell type. An increased percentage that is due to an absolute decrease in another cell type is not an increase at all. |
| Mature
neutrophils have a lobulated nucleus, but when demand is high immature cells with an unlobulated band nucleus may be released into circulation. They function as phagocytes and are important in infectious conditions and in inflammation. Increased neutrophil counts (neutrophilia) are caused by inflammation, bacterial infection, acute stress, steroid effects, and neoplasia of the granulocytic cell line (granulocytic leukemia can be difficult to differentiate from a simple
neutrophilia without special stains or bone marrow biopsy). Decreased neutrophil counts (neutropenia) are caused by viral infections, toxin exposure (including foodborne toxins), certain drugs (eg, carbimazole and methimazole), autoimmune destruction of neutrophils, bone marrow neoplasia not involving the granulocytes, and bone marrow aplasia. |
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Eosinophils are characterized by prominent pink-staining granules on a Romanowsky stain. They inactivate histamine and inhibit edema formation. Increased eosinophil counts (eosinophilia) are caused by allergic/hypersensitivity reactions, parasitism, tissue injury, mast cell tumors, estrus, and pregnancy or parturition in the bitch. Some large continental dog breeds (eg, German and Belgian Shepherds, Rottweilers) normally have a relatively high eosinophil count. Extremely
high eosinophil counts (hypereosinophilic syndrome), possibly due to an out-of-control hypersensitivity reaction, and eosinophil leukemia (a form of chronic myeloid leukemia) are also described. Decreased eosinophil count (eosinopenia) is almost always caused by the action of glucocorticoids, either endogenous or therapeutic. |
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Basophils are rare in most species and are characterized by blue-staining granules on a Romanowsky stain. They are closely related to mast cells and, like them, initiate the inflammatory response by releasing histamine. An increased basophil count (basophilia) accompanies eosinophilia in some species as part of the hypersensitivity reaction. |
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Monocytes are large cells with blue-gray cytoplasm, which may be vacuolated, and a kidney bean-shaped or lobulated nucleus. Their main function is phagocytosis, and they are essentially identical to tissue macrophages. An increased monocyte count (monocytosis) may occur in any chronic disease, especially chronic inflammation, and may be very marked in neoplasia. Monocytes also increase as part of the steroid response in dogs. |
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Lymphocytes mainly develop outside the bone marrow in the lymph nodes, spleen, and gut-associated lymphoid tissue. They are the smallest of the white cells, with a round, evenly staining nucleus and sparse cytoplasm. Their primary function is immunologic, including both antibody production and cell-mediated immune responses. Some survive only a few days, but many are long-lived. The number in circulation is a balance between populations in the blood, lymph, lymph nodes,
and splenic follicles and does not necessarily reflect changes in lymphopoiesis. An increased lymphocyte count (lymphocytosis) may occur for physiologic reasons, especially in cats, but significant increases usually indicate leukemia. Immature or bizarre cells may also be recognized. Decreased lymphocyte counts (lymphopenia) are usually due to an effect of corticosteroids, either endogenous (stress or Cushing’s disease) or therapeutic and may also accompany neutropenia in some viral
infections, especially the parvoviruses. Lymphopenia may also be a feature of solid-organ lymphosarcomas, when leukemia is absent. |
