The function of the immune system is to identify and eliminate foreign viruses, bacteria and fungi. In some cases, the immune system forms a reaction against a seemingly harmless particle. This abnormal reaction is termed as an allergy. Allergies commonly take the form of conditions such as bronchial asthma, hay fever, allergic sinusitis and food allergies. Allergies may sometimes be dangerous and life threatening depending on the person's reaction to that substance.
Genetics of Allergy
Genetic factors undoubtedly play a role in allergies. A genome-wide study of allergies was published by researchers at Johns Hopkins University in 2011 and identified a variety of genes thought to play a role in the formation of allergies. Environmental factors play a role as well. Some theories behind asthma suggest that a lack of childhood infections due to increased hygiene may predispose the immune system towards abnormal reactions. Allergies are thought to originate from reactions that repel worm and parasite infestations.
Abnormal Immune Response
When the body is first exposed to an unknown substance, cells of the immune system, called helper T cells, become activated. They release a variety of signals that activate antibody-producing cells. In a normal immune response, antibody-producing cells make antibodies such as IgM or IgG, but, in this abnormal reaction, they instead produce an antibody known as IgE. The IgE secreted into the blood can bind to specific cells found throughout the body, called mast cells. Once this occurs, the body has become sensitized to this foreign antigen. The stage is set for an allergic reaction.
Allergic Reaction
Upon the next exposure, the irritant can bind to the IgE on mast cells. The mast cells become activated and start to secrete a variety of chemicals that trigger allergic reactions. Among these, histamine is the most important. Histamine is responsible for nearly all the classical symptoms of allergies, including constriction of the airways, dilation of the blood vessels and congestion of the nose and sinuses. Because of the pivotal role of histamine in this reaction, many symptoms of allergies can be treated with antihistamines.
Delayed Hypersensitivity
Another type of hypersensitivity is known as delayed hypersensitivity and works by an alternate mechanism. The symptoms to this kind of allergy often appear 24 to 72 hours following the exposure to the antigen. Common manifestations of delayed hypersensitivity are rashes that occur following exposure to poison ivy, poison oak or nickel contaminants in jewelry. In these types of reactions, a different kind of immune cell, known as a macrophage, becomes abnormally reactive. Macrophage cells slowly reach the site of exposure and attack the foreign particles. Because this process occurs gradually, the manifestations of these reactions can last for days.
References
- "Proceedings of the American Thoracic Society"; Genetic Studies on the Etiology of Asthma; K.C. Barnes; May 2011
- "Davidson's Principles and Practice of Medicine"; Nicholas A. Boon, et al.; 2010
- "Robbins & Cotran Pathologic Basis of Disease"; Vinay Kumar, et al.; 2009
