Eosinophils are normally of similar size to neutrophils or may be slightly larger. The nucleus is bilobed to multilobed and has coarse, dark, clumped chromatin. The distinctive feature of eosinophils is their bright pink cytoplasmic granules (containing major basic protein, eosinophil peroxidase, and other substances). The morphology of these granules varies slightly with species. In the dog, granules are pink-red and variably- sized in most breeds. Greyhounds and some other breeds may have clear granules due to altered stain uptake, so eosinophils may be more difficult to distinguish (called "gray" or "vacuolated" eosinophils). In cats, the granules are small and rod-shaped, in horses they are very large and bright pink (see image above), and in cattle they are small and uniformly-sized. Eosinophils are produced in marrow, circulate in blood for a few hours, and migrate into tissues where they survive for several days. Increased production of eosinophils is mediated by factors produced by some activated T lymphocytes. Corticosteroids decrease blood eosinophil numbers but increase the marrow pool of eosinophils. Increased numbers of circulating eosinophils may be seen in hypersensitivity reactions, as with certain forms of parasitism and allergic conditions. The presence of mast cell tumors in an animal may also be associated with eosinophilia. Basophilia sometimes occurs concurrent with eosinophilia. Idiopathic "hypereosinophilic" syndromes are seen infrequently in cats. Eosinophilia
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