Eosinophils, or "acidophils", are leukocytes. They are one of the immune system components which combat parasites and certain infections. As with mast cells and basophils, they part causes of allergy and asthma. Eosinophils are round cells with a lobed nucleus and granules which turn red when stained. These granules are packed with proteins that can be poured out to help destroy invaders.