What Causes Asthma?
Although asthma may be defined by a lot of factors, its cause remains uncertain. At this time, it appears that the following factors are involved.
Heredity
Heredity certainly plays a major role. Asthma and allergy often occur in families. Geneticists have located a gene of 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 chemical are the asthma mediators. Further research will undoubtedly produce a more detailed explanation for the genetic basis of asthma.
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, in the meantime, involves the 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 (which may also be called an allergen) is a substance capable of provoking the immune response.
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 mas cells, that are known to store and release many irritating chemicals involved in the production of the asthmatic reaction.
The second major immune response to asthma is the production of antibodies known as immunoglobulins, which is stimulated by substances released by the activated lymphocytes. One type, Immunoglobulin E, may be produced by inhaling a specific foreign substance, like ragweed. When 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
Allergy is the leading cause of asthma. In may patients, allergens, activated lymphocytes, mast cells and IgE all play major roles in the immuse response that produces asthmatic reactions. However, asthma may also occur without allergy. In nonallergenic patients, doctors believe the immune response may be triggered by infection and certain viruses.
The Environment
Environment 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.
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 the narrowing of the bronchial passages.
At this time, the specific causes of asthma and the signals that are involved in starting the chain reaction that produces the condition are unknown. In the past year, promising research has shown that asthma is an inflammatory disease with features similar to other illnesses, like arthritis. This breakthrough is likely to lead to further understanding of the causes of asthma and will likely form the basis for greater advances in treatment.