The Allergy Connection - What Causes Asthma: Asthma

Content provided by the Faculty of the Harvard Medical School

The allergy connection

When most people think of allergies, they're likely to think of sneezing and a runny or stuffy nose; red, itchy, watery eyes; or perhaps dry, red, itchy skin. But one theory about asthma is that it is also an allergic reaction — in this case, affecting the bronchial tubes.

In fact, allergic rhinitis (allergies of the nose), allergic conjunctivitis (allergies of the lining of the eyes), and allergic dermatitis (allergies of the skin, also called eczema) frequently occur together in various combinations both in individuals and in families, and they often occur together with asthma.

All these conditions share a common mechanism: They result from a specific type of allergic reaction. The tendency to have this specific type of allergic reaction is called atopy, and people who have this tendency are said to be atopic. Asthma accompanied by allergies is often referred to as atopic asthma or extrinsic asthma. The vast majority of children with asthma, and probably at least half of the adults with the disorder, have the atopic variety.

The allergic basis of asthma has provided insight into which proteins and cells are involved in the inflammation of the bronchial tubes. People who have allergies make a special kind of protein, called an antibody, that is precisely shaped to recognize and attach itself firmly to a particular allergen. This antibody belongs to the family of immune defenders called immunoglobulins. The defenders that are specifically designed to recognize allergens are referred to as immunoglobulin E (IgE). Let's say you are allergic to dust mites; you make IgE antibodies that recognize the dust mite allergen.

The IgE antibodies do not roam freely in your breathing tubes but are firmly attached to immune system cells called mast cells. These cells are located mainly in parts of the body that regularly encounter substances from the outside world: the skin, intestinal tract, lining of the eye (conjunctiva), nose, and breathing tubes.

In people with allergic asthma, the surface of each mast cell is coated with firmly attached IgE antibodies. All remains quiet until the IgE antibodies recognize and attach themselves to an allergen. Then, within seconds, an explosive reaction takes place (see Figure 3). The mast cell makes and releases a barrage of chemicals that carry out the inflammatory reaction. These chemicals include histamine (which you may be aware of if you are familiar with antihistamines for treating allergies), leukotrienes, and many others.

Together these inflammatory chemicals cause blood vessels to leak fluid, producing swelling in the breathing tubes. They stimulate the walls of the breathing tubes to secrete mucus. They also make the muscles surrounding the breathing tubes contract, narrowing these passageways. As if that weren't enough, mast cells also call in reinforcements, other cells involved in allergic inflammation that travel from the blood to the site of the allergic reaction and make things worse. The most important of these reinforcements are blood cells called eosinophils. Like mast cells, they release chemicals that cause the airways to narrow.

Figure 3: Allergic asthma If your asthma is triggered by exposure to an allergen, mast cells located along the surface and inside the walls of the airways are ready to react when the allergen is encountered. After IgE antibodies located on the surface of the mast cell recognize and attach themselves to an allergen, the mast cell makes and releases a barrage of chemicals, such as histamine, that produce an inflammatory reaction.

Chronic inflammation

In asthma, the allergic inflammatory process never really stops. An important medical discovery was that some inflammation is present in the bronchial tubes of people with asthma even when they feel well and when their breathing is normal. Medical researchers performed experiments in people with asthma to sample (biopsy) small pieces of the walls of the bronchial tubes. The samples were taken at times when the research participants were free of asthma symptoms. Yet in the biopsied tissues, scientists found evidence of persistent inflammation of the bronchial tubes.

This suggests that airway inflammation in asthma is always present, at least to some degree. The inflammation may be so mild that it does not cause narrowing of the bronchial tubes. But the persistent presence of this inflammation is probably a major reason that the bronchial tubes are twitchy, or capable of narrowing abnormally. By understanding more about the multiple steps involved in this type of inflammation, physicians have developed treatments aimed at keeping it under control (see "Medications to treat your asthma").

Last updated:	September 27, 2007