Eosinophils are specialized cells of our immune system. They are identified by their distinctive granules that stain red when treated with an acidic reagent, eosin, that gave them their name. Eosinophils are typically rare in our blood and tissues, accounting for about 3% of our white blood cells. Their biological roles are poorly understood, but recent studies suggest that eosinophils are involved in regulating our fat metabolism, repairing certain tissues, and helping us fight different infections and cancers.