Asthma Health Article

Definition

Asthma is a chronic inflammatory disease of the respiratory system that causes breathing difficulty. Asthma comes from the Greek word for panting. The disease is an over-responsiveness of the respiratory system to stimulating factors. It is characterized by repeated, temporary episodes of constriction and inflammation of the airways and lungs, along with excess mucous production. Asthma causes wheezing, coughing, and shortness of breath. Asthma attacks are characterized by severe difficulty breathing, especially when exhaling. Severe attacks that are left untreated may become fatal. An individual with asthma may be completely without symptoms between attacks.

Description

Asthma is a chronic, lifelong disease that affects the complex network of air passageways of the respiratory system. People with asthma may experience from mild discomfort to life-threatening attacks that require immediate emergency treatment. The respiratory system is made up of bronchial tubes (airways) and the lungs. Asthma involves the inflammation of the bronchial tubes and lining of the lungs. The inflammation causes the airways to be overly sensitive to irritating factors, which cause constriction and obstruction to the passage of air into the lungs. Asthmatics also produce excess amounts of mucous in the respiratory tract. Mucous is a normal component of respiratory function that aids in carrying irritating particles up and out of the respiratory system to be expectorated (coughed up) from the body. Asthmatics produce excessive, abnormally thick mucous that interferes with breathing and contributes to the problem. Severe asthma attacks can be fatal. Persistent or chronic inflammation of the airways can cause permanent damage, or airway "remodeling," and reduce lung function so that breathing becomes less efficient even outside of asthma attacks. Asthmatics may experience chronic wheezing, coughing, shortness of breath, and a feeling of a tightening of the chest. Medication and careful management of the disease is often necessary for maintaining normal function.

Chronic asthma has both a genetic and an environmental component. Research has demonstrated that some individuals inherit a strong genetic predisposition for asthma that can be triggered by a variety of environmental factors. Stimuli for triggering asthmatic symptoms include repeated exposure to irritants, such as dust mites, pet hairs, and tobacco smoke. These types of stimuli are considered allergens, or particles that trigger an allergic response. Asthma may also be induced by exercise, especially in cold climates where the respiratory system has to work harder to warm and moisten inhaled air. Some asthmatics only experience symptoms during viral infections. Asthma may also be stimulated by emotional stress. Both physical and psychological factors may precipitate an asthma attack.

Genetic profile

Asthma is a complex heritable disease in which a number of different genes contribute to asthmatic predisposition. While genes may cause a predisposition to asthma, actual asthma attacks are triggered by stimulating environmental factors. It has been clearly established that asthma tends to run in families. Research demonstrates increased risk of developing asthma for children of asthmatics. Studies also show that identical twins are more likely to share a genetic predisposition for asthma than are fraternal (non-identical) twins.

According to the National Institutes of Health (NIH) in 2005, chromosomes 5, 6, 11, 14, and 12 have all been implicated in asthmatic predisposition. However the relative role each of these genes has in asthma predisposition is not clear. One of the most likely candidates for further investigation is chromosome 5. Chromosome 5 is full of genes-encoding molecules involved in the inflammatory response that characterizes asthma.

Research studies show that specific symptoms experienced by asthma patients, such as the inflammation of the airways and lungs, are initiated by the action of genes that regulate the activity of the immune system. In other words, these genes control how the immune system responds to the presence of substances that can potentially trigger asthma symptoms. Like a microscopic army, the immune system consists of a wide array of specialized cells that work together to neutralize threats to the system. Antigens are any foreign agent invading the body that triggers such an immune response. Antigens include disease-producing organisms such as viruses, toxic chemicals in the environment, or allergens such as animal dander and dust mites. In response to the identification of foreign antigen particles, some immune cells produce antibodies to attack specific types of antigens. This immune response occurs after an initial encounter with an antigen and is known as a primary immune response. The immune system recognizes past contact with specific antigens by maintaining specific levels of the antibodies customized to attack specific antigens. When the same antigen is encountered again, the specific antibodies that have been maintained in the body multiply and mount a stronger immune response than the primary response. This process is known as the secondary immune response.

One of the specific antibodies produced in response to allergens is a protein known as immunoglobulin E (IgE), encoded by chromosome 5. In a normal inflammatory response, IgE recognizes foreign antigens and initiates immune reactions against the antigen by binding to other immune cells such as mast cells. Mast cells release chemical mediators that contribute to inflammation directly, but also recruit more immune cells to the site of inflammation. The recruited immune cells also release mediators of inflammation, such as histamine, that amplify the response and cause inflammation. Chromosome 5 encodes for multiple components of this immune response. In asthmatics, the IgE mast cells are highly excitable, making them hypersensitive to stimulation. When foreign antigens are breathed into the respiratory system, the entire inflammatory process, including the recruitment of other immune cells that release histamine, becomes exaggerated, resulting in asthma.

Research indicates that asthmatics produce higher levels of IgE antibodies, more hyperactive mast cells, and higher levels of consequent histamine than non-asthmatics. Histamine is a type of chemical signal that initiates the inflammatory response. Histamine stimulates the dilation of blood vessels walls and makes them more porous. As a result, blood fluid and proteins leak out of the blood vessels and into surrounding tissue, causing the swelling and reddening typical of inflammation. Inflammation involves increased blood flow to affected tissues to allow the passage of the recruited immune cells from the blood into the affected tissues. The immune cells may then dispose of the foreign particles. While this response is designed to defend the tissue from foreign invasion of harmful particles, an exaggerated response can be dangerous. In asthma, the resultant inflammation, along with the reactive constriction of the muscles in walls of the bronchial airways, narrows the air passages and causes an asthma attack.

Another component of the immune defense is the production of nitric oxide gas (NO) by an enzyme called inducible nitric oxide synthase (iNOS). Cells lining the bronchial tubes contain this enzyme that produces NO in response to chemical signals released from immune cells. Asthmatics produce an abnormally high level of iNOS in their respiratory cells than do non-asthmatics. Asthmatics have higher levels of NO in their lungs and bronchial tubes that contribute to the disease.

While chromosome 5 is implicated in asthma, there is conflicting evidence as to whether the genes responsible for the hyperactivity of the immune response in bronchial passages are distinct from the genes that regulate the action of the immune system. However, a region of chromosome 5 involved in the regulation of the immune system has been named bronchial hyperresponsiveness-1 (BHR1). Research on the BHR1 region is currently being performed by the NIH, in addition to other genetic regions. Another possible contributing factor for the overproduction of IgE antibodies could be a lack of exposure to common childhood illnesses. For example, cold viruses and other respiratory illnesses stimulate the human immune system to produce a certain type of immune cell, called a helper T cell, which specifically targets these disease agents. However, in the absence of stimuli, the immune system instead produces another type of helper T cell that initiates the production of the IgE antibody.