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. Avian basophils contain many small purple granules, similar to basophils of some mammals.