Some of the blood cells of non-mammalian vertebrates are very different from
mammalian cells in form but have comparable functions. The heterophil is the avian
equivalent of the neutrophil. Heterophils in birds have segmented nuclei that are partly
obscured by the large red-orange granules that pack the cytoplasm. Mature granules are
fusiform and many are slightly refractile. Immature heterophils have rounder granules and
slightly blue cytoplasm. Avian eosinophils are slightly smaller than heterophils and their
granules are round and a slightly different shade of orange. The granules of eosinophils in
psittacine birds are light blue. Examples of these two types of avian eosinophils are shown
here. The eosinophil on the left is from an African gray parrot and has large globular blue
granules; the arrow points to the nucleus. Avian basophils contain many small purple
granules, similar to basophils of some mammals. Avian lymphocytes and monocytes are
similar to mammalian cells.
Mature red cells in birds are large oval cells that contain oval nuclei. Immature red cells are rounder, have less oval to round nuclei, and polychromatophilic cytoplasm. Polychromatophilic red cells are fairly numerous in some avian species in the absence of anemia. The thrombocyte is the functional equivalent of the mammalian platelet. Thrombocytes in birds are small cells with round nucleus, very condensed chromatin, and small rim of gray cytoplasm. A few red granules are visible in some thrombocytes. Partial activation results in clumping of thrombocytes. Thrombocytes are distinguished from lymphocytes by their smaller size and gray, rather than blue, cytoplasm.
Protozoan organisms of two genuses, Hemoproteus and Leukocytozoon, are commonly seen as parasites of avian blood cells.