Definition of asthma
demonstrated by the sudden, severe attacks patients can experience when exposed to substances such as pollen, animal dander, dust, and fumes. This hyper reactivity forms the basis for bronchial provocation or challenge testing that is used by physicians to diagnose asthma in patients whose illnesses do not fit easily into the other definitions.
What Causes Asthma?
Although asthma may be defined by the factors noted above, its cause remains uncertain. At this time it appears several factors are involved.
Heredity certainly plays a major role: asthma and allergy often occur in families. Geneticists have located a gene on chromosome 11 that is strongly associated with allergy and speculate that several other genes may also be involved. One study suggests that a variant gene may direct the immune system to “overreact” to allergic stimuli by allowing a protein known as
Immunoglobulin E (IgE) to “lock on” to the surface of allergy cells called mast cells. When IgE reacts with allergy substances known as allergens, the mast cell disintegrates, releasing irritating chemicals that cause inflammation. These chemicals are the asthma mediators. Further research will undoubtedly produce a more detailed explanation for the genetic basis of asthma.
The Immune System
The immune system also plays a major role in the development of asthma. The immune system has two basic branches: cellular and humoral. Cellular immunity involves white blood cells called lymphocytes that can be provoked or “sensitized.” An example of this would be the body’s rejection process against a transplanted organ. Humoral immunity involves production of substances called antibodies that circulate in the blood. An example would be how the body reacts to a vaccination by producing antibodies. An antigen (may be called an allergen) is a substance capable of provoking the immune response
Lymphocytes, Mast Cells, and Eosinophils
In asthma, the immune system is provoked in two ways. First, the cellular elements are mobilized and activated. Microscopic studies of the lining of the bronchial tubes in asthma have revealed increased numbers of lymphocytes. These cells produce substances that result in an increase in the number of mast cells that are known to store and release many irritating chemicals involved in production of the asthmatic reaction. These chemical substances or mediators of asthma produce inflammation. Another active cell that is “recruited” by lymphocytes found in the inflamed bronchial lining is the eosinophil. Large numbers of these cells may also be found in the blood of allergic and asthmatic individuals
The second major immune response in asthma is the production of antibodies known as immunoglobulins, which is stimulated by substances released by the activated lymphocytes. One type, Immunoglobulin E or IgE, may be produced by inhaling a specific foreign substance such as ragweed. When the IgE attaches to the surface of the mast cell, a process is initiated that leads to release of the “asthma chemicals” and an ensuing asthmatic reaction.
Allergy is the leading cause of asthma. In many patients allergens, activated lymphocytes, mast cells, eosinophils, and IgE all play major roles in the immune response that produces the asthmatic reaction. However, asthma may also occur without allergy. In nonallergic patients doctors believe the immune response may be triggered by infection.
Viral infections in susceptible individuals have been thought to be potent triggers for the development of asthma. Researchers have recently demonstrated that viruses may cause human immune system cells to produce IgE. Animal research has shown that viruses are capable of altering the nervous impulses that stimulate the bronchial tubes. The altered nerve impulses may then produce constriction in the bronchial tubes. Susceptible patients with viral bronchial infections may become “sensitized” and display all the features noted in the definition of asthma.
Environmental irritants, such as cigarette smoke, pollutants (ozone, particulates), dust, and chemicals and proteins found in the home and the workplace are also considered capable of provoking the asthmatic response. These irritants may account for large numbers of asthmatic attacks each year and may also, in part, explain an increase in the number of asthma cases.
The Nervous System
Another possible cause of asthma is a dysfunction of nerve receptors or endings in the muscle surrounding the bronchial tubes that produces constriction of the air passage. Research has shown that an imbalance may exist in the nervous system that supplies the bronchial tubes of asthmatic individuals. This inborn error may shift the balance of forces toward those nerve signals that promote narrowing of the bronchial passages.