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What is an Allergic Reaction?

By Brendan McGuigan
Updated Mar 03, 2024
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An allergic reaction is simply the human body’s way of trying to fight off a foreign invader it perceives to be dangerous. It is a response of the body’s immune system, and although not all aspects of how exactly an allergic reaction functions are understood, the basic function is fairly simple.

The body is wired to detect what are known antigens, which are foreign objects that the body has decided might be harmful. Once these antigens are detected, a reaction is triggered to get them out of the body as quickly as possible, before anything bad can happen. The trick is that the body isn’t always that good at figuring out what is a dangerous invader and what isn’t. When it makes a mistake, and over-reacts to a foreign invader, the triggered immune response is called an allergic reaction.

An allergic reaction is often the least serious form of immune system dysfunction, and is classed as a Type I hyper-reaction. In extreme cases, however, Type I reactions can trigger anaphylaxis that can lead to death. Type II reactions are also known as cytotoxic hypersensitivity, and can trigger the body destroying its own cells, such as in thrombocytopenia or autoimmune hemolytic anemia. Type III reactions are often called immune complex diseases, an include Arthus reaction and serum sickness. Type IV reactions are delayed reactions, and the most common Type IV reactions are contact dermatitis reactions, such as poison oak or poison ivy.

An allergic reaction can be to virtually anything found in the world. Dust is an incredibly common mild allergy, as is pollen and many different types of food. Bee stings and ant bites, various plants, and some common medication are also common causes of allergies. In some cases, as with many people’s tree nut or bee sting allergies, they may be quite severe. In these cases the body goes into an extreme allergic reaction which can induce shock and may prove fatal.

When the body first encounters a new allergen, it triggers a type of T cell, which then go off an interact with the B cells responsible for producing antibodies. The B cells then begin to produce the antibody IgE in large quantities. The IgE then makes its way through the body, and links up with mast cells and basophils, two types of immune cell. Newly coated with IgE, which acts as a sort of messenger, these cells now know what the allergen looks like, and to be on the lookout for it. When the cells run into the allergens, they’re now able to capture them.

At the same time, they release various mediators, which cause the outward effects of the allergic reaction. Depending on the allergen itself, the allergic reaction may take different forms. The cells may, for example, release histamine, making the skin itch severely. They may also release chemicals that trigger mucus production, making the nose runny. They can also cause vasodilation, which can cause asthma if it takes place in the lungs. After running their course, the reaction may subside, in which case it was simply an acute response, or it may change form and continue as other cells, such as neutrophils or macrophages make their way to the allergen clumping.

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Discussion Comments
By anon132082 — On Dec 05, 2010

I am allergic to shellfish and now doc says avoid fish too. Would there be danger in using skin product which contains marine collagen? thanks. --bundle

By BoatHugger — On Aug 29, 2010

My cousin’s son has a horrible allergy to bee stings. He has severe allergic reactions. He has to carry an Epi-pen with him everywhere he goes. In the event of a bee sting, he can administer the Epi-pen treatment himself. It contains the correct dose of Epinephrine to prevent an anaphylactic reaction.

By calabama71 — On Aug 29, 2010

@dill1971: If the allergic reaction is on your skin, you can try hydrocortisone cream to relieve the redness and the itching. Wet cloths and ice packs are also beneficial. Benadryl also has a cream available.

By wesley91 — On Aug 29, 2010

@dill1971: Treating allergic reactions depends greatly on what the reaction was to and how severe the reaction was.

It the allergic reaction is mild, consisting of sneezing, coughing, runny nose, and itchy or watery eyes, it can more than likely be treated at home with an antihistamine.

Antihistamines are very helpful when the allergic reaction is to animal dander, grasses, pollens, and other outdoor triggers. If you have to go to work or school, non-drowsy antihistamines are best because they won’t make you drowsy.

By dill1971 — On Aug 29, 2010

How are these allergic reactions treated?

By wafica — On May 17, 2009

What other legumes can cause favism other than fava beans?

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