Eosinophils are members of the granulocytic class of white blood cells that function primarily in fighting infections of parasites and in allergic reactions. The cells, which are named for the eosin-staining granules they contain, generally comprise one to three percent of the total white blood cell count in a healthy individual.

Persons who exhibit an abnormally large number of eosinophils are said to have eosinophilia. Though eosinophilia may be beneficial at times since the increase in white blood cells helps rid the body of certain parasitic invaders, the condition can also lead to tissue damage as an increasing number of eosinophils accumulate in the body. Asthma patients, for instance, often experience additional injury to the lungs due to eosinophilia.

Eosinophilia is known to have many causes, the most common of which are helminthic (worm) infections and allergic conditions, such as asthma and hay fever. The former is the primary cause worldwide, but the latter is much more prevalent in the United States, Great Britain, and other developed nations. Rarer causes of eosinophilia include cirrhosis of the liver, certain tumor types, such as lymphoma, lung diseases, rheumatoid arthritis, dermatitis herpetiformis and other rare skin conditions, Churg-Strauss vasculitis, and various antibody deficiencies. In some instances, no cause is apparent, in which case the condition is termed hypereosinophilic syndrome.

In instances of eosinophilia that are linked to a specific cause, the symptoms that appear often are related to the underlying ailment. For example, helminth-related eosinophilia is marked by abdominal pain, diarrhea, and fever, whereas an elevation in eosinophil count due to asthma is generally manifested in the form of breathlessness and wheezing. Treatment for eosinophilia generally is targeted at curing the underlying condition or, if no cause is known, involves the administration of oral corticosteroids.

Epidemics of eosinophilia have been known to occasionally occur. In 1981, for instance, numerous cases of eosinophilia appeared in Spain that were eventually officially attributed to contaminated cooking oil and became known as toxic oil syndrome, although there have been many critics of the official governmental findings. This syndrome affected over 20,000 people, claiming the lives of hundreds and leaving thousands of others with long-term disabilities. More recently, a condition dubbed eosinophilia-myalgia syndrome was recognized in the United States. Initially described in New Mexico in 1989, the syndrome, which is characterized by a high eosinophil count and debilitating muscle pain, was found to be connected to a health food supplement, L-tryptophan, which was marketed as a sleep aid. Impurities in batches of the supplement produced by the Japanese petrochemical company Showa Denko Inc. are considered to have been responsible for at least 37 deaths and 1,500 cases of the syndrome, which leads to permanent disability.

Eosinophilia at 20x Magnification - Though it occurs infrequently, hypereosinophilic syndrome can have serious consequences, often adversely affecting the heart, liver, and spleen in addition to causing swelling of the ankles, skin rashes, and other mild symptoms.