Eosinophils Explained: What They Are and How They Function

Eosinophils are a type of white blood cell, or leukocyte, that play a crucial role in our body's defense mechanism. They are primarily known for their involvement in allergic reactions and protection against parasitic infections. This article delves into the anatomy, function, and significance of eosinophils in our immune system.

What are Eosinophils?

Eosinophils are a type of leukocyte or white blood cell, produced in the bone marrow. They make up about 1-4% of the leukocyte population in the blood. However, their concentration is higher in tissues exposed to environmental agents such as the mucous membranes of the digestive and respiratory tracts, the genitourinary epithelia, and the skin connective tissue. Here, they serve as a protective barrier against potential parasite attacks.

Eosinophils, along with Tc lymphocytes, are categorized as cytotoxic leukocytes due to their ability to damage or kill harmful entities. They also belong to the category of granulocytes, which includes basophils and neutrophils, characterized by the presence of numerous small cytoplasmic granules.

The name "eosinophils" stems from the fact that their cytoplasmic granules are stained pink-red with a particular dye called eosin. These granules contain various chemical substances that mediate the defense and modulatory reactions in which eosinophils are involved.

Anatomy of Eosinophils

Eosinophils are typically 10-14 ┬Ám in diameter and have a bilobed nucleus. Their granules have an affinity for red dyes, particularly eosin. Like neutrophils and basophils, eosinophils are polymorphonuclear leukocytes, meaning they have irregularly shaped nuclei with two lobes. They are also granulocytes due to their abundant cytoplasmic granules containing reactive substances capable of killing microorganisms and enhancing inflammation.

Functions of Eosinophils

Eosinophils play a vital role in inflammatory reactions and immune responses against allergens or parasitic infestations. They are equipped to attack parasites that cannot be incorporated due to their size. Their strategy involves adhering to the foreign agent's body and discharging toxic reactive substances against it.

Eosinophils are also particularly active during inflammatory and allergic reactions, where they contribute to the inflammatory process and tissue damage through the release of oxidizing substances and toxic enzymes. However, their cytotoxic substances can sometimes cause tissue damage and discomfort.

Defense Against Parasitic Infections

Parasites, including unicellular protozoa and multicellular invertebrates, can cause persistent and long-lasting infections in humans. Eosinophils play a crucial role in combating these parasites. Inflammatory mediators released by eosinophils can cause contraction of the smooth muscles surrounding the airways and intestines, leading to the expulsion of parasites.

Defense Against Bacterial Infections

While eosinophils are less efficient than neutrophils at killing intracellular bacteria, they still play a role in the body's defense against bacterial infections.

Immediate Hypersensitivity Reactions

Eosinophils can modulate immediate hypersensitivity reactions by degrading or inactivating mediators released by mast cells, such as histamine and leukotrienes, which can cause vasoconstriction and bronchoconstriction.

Production and Life Cycle of Eosinophils

Eosinophils are produced and matured in the bone marrow over 8 to 10 days. After this phase, they enter the bloodstream and migrate into the tissues within 8-12 hours. Here, they remain for a few days without re-entering the circulation. Eosinophils have a longer lifespan compared to other granulocytes.

Normal Values and Variations

Under normal conditions, eosinophils constitute 1-4% of nucleated cells in peripheral blood. However, their number can increase significantly during many diseases, particularly parasitic infestations and allergic reactions, a phenomenon known as eosinophilia. Conversely, a decrease in eosinophils, known as eosinopenia, can be observed in the acute phase of some infectious diseases, anaphylactic shock, and after the administration of corticosteroids.


Eosinophils, while often overlooked, play a vital role in our body's immune response. They serve as a defense mechanism against parasitic infections and contribute to the mediation of inflammatory and allergic reactions. Understanding the function and significance of eosinophils can provide valuable insights into the complex workings of our immune system.

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