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What are eosinophils?

Eosinophils (Eos) are a type of white blood cell, which are the body's first line of defense against infection. They are commonly found in increased numbers with parasitic infections and allergies. When eosinophils arrive to fight a parasite or allergen, they break open and the granules, containing the toxins, spill out. This is called “degranulation”. The toxins, which are meant kill parasites, infiltrate and damage the surrounding tissue.

Eosinophils (as pictured above) have bi-lobed nuclei, and their cytoplasm is filled with approximately 200 large granules containing enzymes and proteins. They stain a bright pink, or magenta color, and the nuclei look blue.

They are present in the blood and in certain tissues. Normally, they only make up 1-3% of the total white blood cells in the bloodstream. Lymphocytes and neutrophils are more common. They are created in the bone marrow, they move to the blood, and then to tissue.

All of the eosinophil granule proteins, including major basic protein (MBP), eosinophilic cationic protein (ECP), eosinophil peroxidase (EPO) and eosinophil-derived neurotoxin (EDN), are capable of causing tissue damage and dysfunction. Eosinophils have also been known to be toxic to other tissue, including in the heart, brain, bronchial, and intestinal epithelial tissue.