By Robert H. Shmerling, M.D.
Harvard Medical School

Chances are good that you have never heard of eosinophils. These cells are not as abundant or famous as red cells or the family of white cells (of which they are one type), but they do circulate in significant numbers in the bloodstream and reside in even larger numbers in the tissues. The name comes from "eos" (from the Greek, "dawn") and "philein" ("to love"), referring to the way these cells attract eosin, a rose-colored stain commonly used in viewing blood cells under a microscope.

Blood Cells: A Primer

Our blood normally contains three types of cells: red cells, platelets and white cells. The main function of red cells is to pick up oxygen in the lungs, deliver it throughout the body and return carbon dioxide back to the lungs for removal. Platelets handle clotting, so that, for example, when you have a cut, bleeding stops within a minute or so. The white cells are primarily involved in fighting infection and in the repair of injured tissues. Several types of white cells perform different functions; for example, one type, the lymphocytes, makes antibodies.

Eosinophils: Worms, Wheezes And Weird Diseases

The exact role of eosinophils is uncertain, but they seem to be important in the body's response to certain "foreign" proteins. Eosinophils tend to increase in the blood and tissues when certain conditions are present, particularly allergic conditions or particular types of infections.

Examples of conditions associated with an increase in eosinophils are:

• Parasitic infections - A good example of a parasitic infection associated with increased eosinophil levels is Strongyloides infection, in which a worm travels through the skin to the bloodstream, lungs and eventually the intestinal tract. This infection is more common in tropical regions outside the United States.
  
  
• Asthma - In this common lung disorder, airways reversibly constrict, often leading to shortness of breath and wheezing; allergic reactions or respiratory infections may trigger asthma.
  
  
• Allergic reactions - Allergic reactions, including those related to medications, are associated with increased eosinophil levels.
  
  
• Rare disorders - Rare disorders related to eosinophils include eosinophilic fasciitis, in which the tissues beneath the skin (called the fascia) are inflamed and scarred, and Churg-Strauss vasculitis, a condition in which eosinophils appear to be an integral part of blood vessel inflammation within the kidneys, lungs, skin and other sites.

Medical students often remember these conditions by the rhyme "worms, wheezes and weird diseases." In each of these conditions, blood counts often demonstrate elevated levels of eosinophils.

Each of these conditions probably has an element of an allergic or inflammatory reaction that stimulates eosinophils. It is probably no accident that the highest numbers of tissue eosinophils are in places where the body comes into contact with the outside world (such as the airways or the intestinal tract). The various manifestations of these illnesses are part of the reaction: wheezing, diarrhea, or inflammation and scarring of the tissues beneath the skin. Often, the foreign protein that triggers an allergic reaction is not known or cannot be identified. Researchers are trying to identify what stimulates this reaction, so that preventive interventions, vaccinations or more effective treatments can be developed.

Although the "weird diseases" demonstrate how eosinophils do not always accompany allergic disorders or parasitic infections, a disease such as eosinophilic fasciitis could someday be linked to an infectious or allergic reaction -- at present, the cause of most of these rare conditions is unknown.

Rarely, eosinophils seem to increase for no apparent reason. In hypereosinophilic syndrome, eosinophils increase dramatically in the blood and tissues, and yet no specific cause or associated illness is present. Over time, the elevated numbers of eosinophils can lead to scarring of the heart and other organs. It may turn out that this is also an allergic response manifested only by an increase in these blood cells. Finally, high levels of eosinophils may also accompany certain types of cancer, including lymphomas and leukemias.

It is likely that eosinophils are important in the body's ability to respond appropriately to certain exposures we all have. Understanding them better is a focus of ongoing research so that the triggers that provoke them and the trouble they cause on their own can be more readily identified, treated and even prevented. Hopefully, you will never develop one of the "three W's" -- but if you do, you will likely have to credit (or blame) the eosinophil for its role in your body's response.

Robert H. Shmerling, M.D., is associate physician at Beth Israel Deaconess Hospital and associate professor at Harvard Medical School. He has been a practicing primary care physician and rheumatologist for 17 years at Beth Israel Deaconess Medical Center. He is an active teacher in the Internal Medicine Residency Program, serving as the Robinson Firm Chief. He also is a teacher in the Rheumatology Fellowship Program.