Eosinophil

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Microscopic appearance of the blood.

Eosinophils are one of the three main cell types within a specific group of leukocytes (white blood cells) known as granulocytes, along with basophils and neutrophils. These cells are characterized by prominent, type-specific cytoplasmic granules that bind neutral, basic, or acidic stains and perform specific functions.

Eosinophils constitute only 1% to 6% of all circulating leukocytes. They are produced by the bone marrow under the influence of various cytokines, including the interleukin 5 (IL-5), interleukin 3 (IL-3), and granulocyte-monocyte colony-stimulating factor (GM-CSF). 

They measure approximately 2-17 μm in diameter and are typically about 1.5 times larger than erythrocytes (red blood cells). Like other granulocytes, eosinophils contain two main types of cytoplasmic granules: azurophilic granules (lysosomes) and specific granules. In routine blood smears, eosinophils exhibit a characteristic bilobed nucleus and abundant large specific granules that stain red or dark pink with eosin, which is the reason for their name.

The eosinophilic specific granules, which are oval-shaped, contain crystal-like cores composed of alkaline proteins, predominantly major basic proteins (MBP). MBPs, along with other peroxidases and enzymes, are toxic to and destroy parasitic worms, helminths, and some viruses.

Eosinophils are terminally differentiated cells with a short lifespan of one to two weeks. They circulate in the blood for roughly 10 to 18 hours and subsequently exit capillaries into tissues, where over 95% of eosinophils reside. These tissue-based eosinophils are typically prevalent in the connective tissue of the intestinal mucosa and in areas of chronic inflammation, such as the lung tissues of people with asthma. Eosinophils can also migrate to other tissues in response to chemotactic signals from inflammatory or allergic responses .

In addition to their significant role in modulating inflammatory responses, allergic reactions and parasitic infections, eosinophils are also involved in clear antigen-antibody complexes from interstitial fluid by consuming them through the process of phagocytosis.