College of American Pathologists Hematology and Clinical Microscopy Resource Committee

Cells in the basophil line have a maturation sequence analogous to the neutrophil line. At the myelocyte stage, when specific granules begin to develop, basophil precursors can be identified. All basophils, from the basophilic myelocyte to the mature segmented basophil, are characterized by the presence of a moderate number of coarse and densely stained granules of varying sizes and shapes. The granules are larger than neutrophilic granules, and most are roughly spherical. The predominant color of the granules in Wright-Giemsa stained preparations is blue-black, but some may be purple to red. The granules are unevenly distributed and frequently overlay and obscure the nucleus. Basophils are the same size as neutrophilic cells, 10 to 15 μm. The N:C ration is 1:2 to 1:3. Basophils are increased in the blood in several states, including myeloproliferative neoplasms, hypersensitivity reactions, hypothyroidism, iron deficiency, and renal disease. Eosinophils are round to oval leukocytes that are generally easily recognized due to their characteristic coarse, orange-red granulation. They are the same size as neutrophilic cells, 10 to 15 μm for mature forms and 10 to 18 μm for immature forms. The N:C ratio ranges from 1:3 for mature forms to 2:1 for immature forms. Their abundant cytoplasm is generally evenly filled by numerous coarse, orange-red granules of uniform size. These granules rarely overlie the nucleus and exhibit a refractile appearance with light microscopy due to their crystalline structure. This refractile appearance is not apparent in photomicrographs or pictures. Due to inherent problems with the color rendition on photomicrographs, which is sometimes imperfect, eosinophilic granules may appear lighter or darker than on a freshly stained blood film. Discoloration may give the granules a blue, brown, or pink tint. Nonetheless, the uniform, coarse nature of eosinophilic granules is characteristic and differs from the smaller, finer granules of neutrophilic cells. Occasionally, eosinophils can become degranulated with only a few orange-red granules remaining visible within the faint pink cytoplasm. In the most mature eosinophilic form, the nucleus segments into two or more lobes connected by a thin filament. About 80% of segmented eosinophils will have the classic two-lobed appearance. Typically, these lobes are of equal size and round to ovoid or potato-shaped with dense, compact chromatin. The remainder of segmented eosinophils will have three lobes, and an occasional cell will exhibit four to five lobes. Eosinophils exhibit the same nuclear characteristics and the same stages of development as neutrophilic leukocytes. Immature …