The Allergic Response. The allergic process, called atopy, and its connection to asthma are not completely understood. It involves various airborne allergens or other triggers that set off a cascade of events in the immune system leading to inflammation and hyperreactivity in the airways. One description is as follows:
- The conductor in an orchestra of immune factors that contribute to allergies and asthma appears to be a category of white blood cells known as helper T cells, in particular a subgroup called Th2 cells.
- Th2 cells overproduce interleukins (ILs), immune factors that are molecular members of a family called cytokines, which are involved in the inflammatory process.
- Interleukins 4, 9, and 13, for example, may be responsible for a first-phase asthma attack. These interleukins stimulate the production and release of antibody groups known as immunoglobulin E (IgE). (People with both asthma and allergies appear to have a genetic predisposition for overproducing IgE.)
- During an allergic attack, these IgE antibodies can bind to special cells in the immune system called mast cells, which are generally concentrated in the lungs, skin, and mucous membranes. This bond triggers the release of a number of active chemicals, importantly potent molecules known as leukotrienes. These chemicals cause airway spasms, overproduce mucus, and activate nerve endings in the airway lining.
- Another cytokine, interleukin 5, appears to contribute to a late-phase inflammatory response. This interleukin attracts white blood cells known as eosinophils. These cells accumulate and remain in the airways after the first attack. They persist for weeks and mediate the release of other damaging particles that remain in the airways.
The Immune Response
Researchers are investigating the role that T cells play in asthma. T cells are white blood cells that are involved in the immune response. Researchers had focused on the T cell called type 2 helper (ThH2) cells. However, a 2006 breakthrough study in the New England Journal of Medicine suggested that a different type of T cell may play a stronger role in asthma than previously thought.
Researchers discovered that these cells, called natural killer T cells, are far more common in the lungs of people with asthma than in the lungs of healthy people. Natural killer T cells are very rare, but researchers found them in 60% of people with moderate-to-severe persistent asthma. While this research is preliminary, it may explain why corticosteroid drugs do not work well for some patients with asthma: Steroid drugs target Th2 and other inflammatory cells, not natural killer T cells. Researchers think that further investigation of natural killer T cells may lead the way to new types of asthma drugs. If these cells prove to be involved in asthma, then drugs that eliminate them might become an important new treatment.
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