What is an allergic reaction?

Paul M. Ehrlich, MD
Allergy & Immunology
The allergic response is one of nature's dirty tricks. It is a perfectly useful immune mechanism. But deprived of its natural prey -- the parasites that plagued our ancestors in the cradles of civilization -- it comes back to target the wrong things, and so deprive its victims of a normal, comfortable life.
Asthma Allergies Children: A Parent's Guide

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Asthma Allergies Children: A Parent's Guide

Asthma and allergies are at epidemic proportions. It doesn't have to be that way. Two experienced pediatric allergists tell everything a conscientious parent needs to know about these conditions,...
A allergic response is a hypersensitive immune reaction to a substance that normally is harmless or would not cause an immune response in most people. An allergic response may cause harmful symptoms such as itching or inflammation or tissue injury.

This answer is based on source information from the National Cancer Institute.
An allergic reaction is a hypersensitivity disorder in which the person's immune system strongly reacts against a normally harmless substance from the environment to produce an inflammatory response. An allergic reaction is activation of white blood cells called mast cells in the blood and tissues that overreact to produce proteins and immunoglobulin E, ultimately initiating the inflammatory response. Essentially your body is activating and fighting against whatever allergic substance or allergen you came in contact with, producing symptoms such as hay fever, red eyes, itchy and runny nose, eczema, hives, or even an asthma attack.
Anthony L. Komaroff, MD
Internal Medicine
The mechanism of an allergic reaction is essentially the normal immune system reacting as it would when confronted with a harmful microbe. But rather than a pathogen sounding the alarm bell, a harmless allergen triggers an unnecessary immune response. The resulting inflammatory response goes into high gear for way too long, which can damage surrounding tissues. The problem is made worse because once learned, or sensitized, the same response kicks in every time the immune system encounters the harmless allergen. And it takes only minuscule amounts of the allergen to set a full-blown allergic reaction in motion.

Although allergies vary in terms of which allergen causes the reaction and where in the body the reaction takes place, most allergic reactions involve immunoglobulin E (IgE) antibodies, the products of plasma cells, descendants of the immune system's activated B cells. In people with allergies, IgE is stimulated upon first exposure to an allergen such as pollen or bee venom. The IgE antibodies bind to a type of immune cell called a mast cell. The mast cells, now armed with IgE antibodies, roam the body, ready to trigger an allergic response the next time the same allergen appears. Mast cells act like guard dogs at the gates, ready not just to sound the alarm but also to swing into attack mode by instigating the body's inflammatory response.

Allergic reaction starts when the mast cells release certain chemical mediators, such as histamines and other agents. These mediators are instrumental in causing the telltale symptoms of an immediate allergic reaction -- swollen and inflamed tissue, sneezing, coughing, wheezing, runny eyes, itchiness, and so on. And once it's revved up, the reaction keeps on going, with more inflammatory cells massing at the site, releasing more powerful chemicals, such as leukotrienes, which cause yet more inflammation. A prolonged inflammatory response can damage tissues, such as those in the airways, and can lead to debilitating chronic diseases such as asthma.
An allergic reaction is the response of the immune system to a foreign substance that enters the body.

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Important: This content reflects information from various individuals and organizations and may offer alternative or opposing points of view. It should not be used for medical advice, diagnosis or treatment. As always, you should consult with your healthcare provider about your specific health needs.