Eosinophils are a blood cell and are easily stained. The name eosinophil means "eosin loving". This, no doubt, refers to their bright pink stained granules following eosin staining. Unfortunately, the term "eosinophil", or "eosinophilic", is also sometimes used to describe other tissue components which stain brightly with eosin. In other words, "eosinophil" is sometimes used as a proper noun, i.e. a name for a blood cell, when the word "polymorphonuclear" may or may not precede it, and at other times it is used as an adjective to describe the stained appearance of other tissue components. In the latter sense, it often has the same meaning as acidophil, meaning that the component stains brightly with acid dyes.

The polymorphonuclear eosinophil has cytoplasmic granules which stain intensely with acid dyes. These may be referred to as "eosinophil", "eosinophilic", "acidophil", or "acidophilic". The terms are largely interchangeable in practice and refer to the same cells. There are other cells which have acidophil (or eosinophil) granues, but they are usually clearly identified and the term "eosinophil", when used alone as a name of a cell, almost invariably refers to the polymorphonuclear eosinophil, whether in blood or tissue. Selective methods for the demonstration of these cells often use eosin, but some common methods use other acid dyes, such as chromotrope 2R or sirius red F3B. In fact, many acid dyes could be used for their demonstration by the addition of a little base to the dye solution in order to suppress much of the background staining.

Eosinophils are usually quite clearly identified in a routine H&E stain especially if the procedure used has a water or ethanol wash after eosin counterstaining to increase contrast between the tissue elements. Eosinophils will usually appear as bright pink, refractile granules in the cell's cytoplasm. Due to their visibility in the H&E stain their selective demonstration is only infrequently required in diagnostic laboratories, although it may be necessary more frequently in research settings.

Although Romanowsky stains on peripheral blood smears show eosinophil granules very well, they are less prominent on tissue sections and either Lendrum''s method or alkaline sirius red are more successful in making them prominent.

Staining Methods
Simple eosin
Lendrum's chromotrope 2R
Sirius red



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