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Eosinophils

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Microscopic view. Red blood cells, stained pink, are distributed across a white background. At the center, an eosinophil cell stands out with its prominent blue and pink granules. Smaller blue platelets appear throughout the frame. The overall composition is a close-up view, capturing the cellular structure in detail.
Figure 1: Eosinophil with bilobed nucleus and red-staining granules in the cytomplasm.

Eosinophils are a type of white blood cell (leukocyte) characterized by the red-colored granules in the cytoplasm.  Eosinophils become elevated in parasitic infections.

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Eosinophils, along with neutrophils and basophils, are granulocytes.  Their cytoplasm contains granules that stain a characteristic red with the commonly used eosin of H&E stain.

Nucleus

The darkly stained nucleus of eosinophils usually has two lobes. These are connected by a thin strand. It is much like the nucleus of basophils.  

Microscopy image. Numerous pink, biconcave red blood cells surround a central, larger eosinophil. The eosinophil's nucleus is segmented, stained a deep purple, and has a grainy texture. The overall coloration of the slide is light-toned on White background. The composition is dense, showcasing the relative concentration of blood cells in a typical smear.
Figure 2: Microscopic view of an eosinophil amidst red blood cells. The distinctive grainy, bi-lobed nucleus is visible.

Cytoplasm

Eosinophils contain granules in the cytoplasm that have a color unlike that of any other leukocyte: red.   These granules are larger than those found in neutrophils.

Function

Eosinophils are one of the two microphages, the other being neutrophils.  Eosinophils are particularly effective at combating parasitic infections.  Their granules contain a mixture of cytokines that contribute to amplifying inflammation. 

Eosinophils can participate in allergic reactions, along with basophils.  Recently, asthma drugs targeting the effects of eosinophils have made an appearance on the market of asthma drugs.

Confused With

Eosinophils and basophils have similar nuclei, but can always be differentiated by the color of the granules in their cytoplasm.    

All Figures: Slide 86X Human blood smear, Giemsa stain, 86X scan from hematopathology normals collection
All by University of Michigan Histology, licensed under CC BY-NC-SA 3.0.

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Erythrocytes
Platelets
Basophils
Neutrophils
Eosinophils
Monocytes
Lymphocytes

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