Transmission electron micrograph of eosinophils. Eosinophils are a variety of white blood cell and one of the immune system components responsible for combating multicellular parasites and certain infections in vertebrates. Along with mast cells, they also control mechanisms associated with allergy and asthma. They are granulocytes that develop during hematopoiesis in the bone marrow before migrating into blood. Magnification unknown.