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An allergen is any antigen that causes a reaction in sensitive individuals and is not a parasite. An allergen can be almost anything, so long as it provokes an allergic response in a person to something other than a parasite. Generally, the response that comes about when an allergic individual comes into contact with an allergen, the IgE response, is triggered only to destroy parasites that have invaded the body. Allergies are the result of the body identifying a non-parasite as the same sort of danger, and so taking the same response to it.
Ultimately, anything can be an allergen, and there is nothing that is a truly universal allergen. Some things are seen as common allergens, while others are relatively rare. Similarly, some things that are an allergen pose a life-threatening risk in individuals with a high response, while other allergies may be a simple inconvenience. People whose bodies recognize the excretion of dust mites as an allergen, for example, will generally just feel an itching or perhaps slight congestion. So while dust mite excretion is present virtually everywhere, even those individuals who are responsive to it are almost never in any danger.
On the other hand, many people who are allergic to bee stings may experience anaphylaxis if they are stung. This can kill them if they are not treated very quickly, requiring these individuals to carry with them some sort of epinephrine, to halt the allergic reaction before it gets out of control. Some people have similarly strong responses to common foods and ingredients, which makes eating processed food something of a tricky proposition for them.
The Food and Drug Administration in the United States identifies eight common foods that are allergens, and they have to be labeled if they are in products, or if the product is processed on the same machine as food with any of those ingredients. These ingredients are: wheat, milk, peanuts, tree nuts, eggs, soy, sulfites, shellfish, and fish. These eight were chosen by the FDA because a relatively large portion of the population has some degree of allergic reaction to these ingredients.
Other foods that people’s bodies may see as an allergen include beans, corn, pumpkin, and celery. In addition to bees, many people are also allergic to wasps or mosquitoes. Similarly, many types of mold provoke an allergic response in people, as do pollens from a number of common plants, including ragweed, birch, timothy-grass, poplar, and nettle. And a number of animal byproducts are an allergen, especially the dander of cats and dogs, and wool.
Sometimes, the body will even identify a drug intended to be helpful as an allergen. For example, many people are allergic to penicillin, so that they have to have an alternative treatment, otherwise their body will respond negatively to being injected with penicillin. The entire class of sulfonamides, usually just called sulfa drugs, is another common allergen. Since the consequences of taking these drugs if they are an allergen can be deadly, most hospitals and doctors will check to make sure the patient isn’t allergic to common drugs, and more and more doctors steer clear of these common allergens if it’s easy to do so.