Eosinophils are a type of white blood cell. They differ from other white blood cells in that they have a grainy texture when looked at under a microscope. Because of this, they are classed as a type of granulocyte. The grains found in eosinophils are orange-red when stained.
Eosinophils are made in the bone marrow and travel to other points in the body to attack and destroy invaders, such as parasites.
These white blood cells make up around 1 to 6 percent of the body's white blood cells, or leukocytes, and usually circulate around the body for about eight to 12 hours. If they are not stimulated, eosinophils can live in tissue for close to two weeks. They are abundant in proteins and digestive enzymes, including major basic protein, eosinophil cationic protein, eosinophil peroxidase and eosinophil-derived neurotoxin. These proteins help fight infection, but they are destructive to the tissues of both the invader and the host.
When the eosinophil count goes above 500 eosinophils per microliter, the person is said to have eosinophilia. This is a complication of such disorders as certain cancers, skin diseases, rheumatoid arthritis and inflammation of the esophagus. Eosinophilia can also be a side effect of certain drugs. However, the condition is mostly seen in people who suffer from allergies or asthma.