Eosinophils Under The Microscope

Observation and Discussion

Essentially, eosinophils develop from the multipotent hematopoietic stem cells that reside in the bone marrow. While they are some of the more rare leukocytes (making up between 1 and 4 percent of the total white blood cells in circulation) they play an important role in immunity particularly as initiators and propagators of various inflammatory responses during an infection as well as in adaptive immunity.

In particular, eosinophils play an important role in defending the body against infections and parasites that invade the body. A variety of studies have shown these cells to be multifunctional given that they participate in a variety of inflammatory processes that involve bacterial, allergies, viral infections and injury of the tissue among others.

When triggered, they respond in a number of ways including:


  • The secretion of a variety of cytokines (IL-2, IL-4, IL-5,  etc)
  • Transforming growth factor
  • Chemokines
  • Lipid mediators
  • Leukotriene


One of the best ways to view and identify eosinophils involves the use of Romanowsky-stain such as the Giemsa stain. This involves preparing a blood smear for microscopy.



  • Place a few drops of blood on to a clean glass slide (blood should either be obtained by pricking the finger tip or blood that was stored in an EDTA tube)
  • Using another clean slide or cover slip at an angle, spread the drop of blood in order to create a thin film of blood on the slide
  • Allow the preparation to completely dry
  • Fix the film using methyl alcohol (methanol) by placing the slide in a jar with methanol for about 3 minutes - This allows for the morphology of the cells to be preserved


Giesma Stain


To stain the preparation:


  • Fill a beaker or staining jar with the stain
  • Immerse the slide in to the jar
  • Allow the slide to stain for about 40 minutes
  • Remove the slide from the staining jar and wash with water
  • Allow the slide to dry before viewing under the microscope
  • Use oil immersionwhen viewing the slide under 1000x

See also Cell Staining in Microscopy Guide


When viewed under the microscope, eosinophils will appear spherical with a light purple body (of the cytoplasm) and dark blue/purple nucleus with two lobes (bi-lobed). Students will also be able to see course granules in the cytoplasm of the cell that may appear reddish orange (as dark spots)

Eosinophils Discussion

Like basophils and neutrophils, eosinophils are granulocytes, which means that they have granules in their cytoplasm. The large granules contain enzymes and proteins that serve different functions with regards to immunity.

As previously mentioned, once triggered, they release a variety of molecules that contribute to the eosinophil response.

For the most part, these molecules present pro-inflammatory effects including:

  • The up-regulation of adhesion systems
  • Cellular trafficking modulation and activation
  • Regulating vascular permeability
  • Constriction of smooth muscle


These cells also function by initiating antigen-specific immune responses through a process where they behave like antigen presenting cells. By releasing toxic proteins and lipids mediators from the granules, eosinophils are also capable of not only inducing tissue damage, but also degrading other foreign organisms in given tissue.

See Also:  Monocytes, Neutrophils, Basophils, Lymphocytes

See Blood Smear Main Guide

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