An allergen is any antigen that causes a reaction in sensitive people and is not a parasite. An allergen can be almost anything, as long as it causes an allergic response in a person to something other than a parasite. In general, the response that occurs when an allergic person comes into contact with an allergen, the IgE response, is activated only to destroy the parasites that have invaded the body. Allergies are the result of the body identifying a non-parasite as the same type of danger and therefore receiving the same response.
Ultimately, anything can be an allergen, and there is nothing that is a truly universal allergen. Some things are considered common allergens, while others are relatively rare. Similarly, some things that are allergenic pose a life-threatening risk to high responders, while other allergies may be just an inconvenience. People whose bodies recognize dust mite excretion as an allergen, for example, will usually feel itchy or perhaps slightly congested. So while dust mite shedding is present just about everywhere, even the people who respond to it are almost never in danger.
On the other hand, many people who are allergic to bee stings may experience anaphylaxis if stung. This can kill them if not treated very quickly, which requires these people to carry some type of epinephrine with them, to stop the allergic reaction before it gets out of control. Some people have equally strong responses to common foods and ingredients, making eating processed foods a difficult proposition for them.
The US Food and Drug Administration identifies eight common foods that are allergens, and they must be labeled if they are in products or if the product is processed in the same machine as foods with any of those ingredients. These ingredients are: wheat, milk, peanuts, tree nuts, eggs, soybeans, 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 can see as allergens include beans, corn, squash, and celery. In addition to bees, many people are also allergic to wasps or mosquitoes. Similarly, many types of mold elicit an allergic response in people, as do pollens from several common plants, including ragweed, birch, timothy grass, cottonwood, and nettle. And a number of animal by-products are allergens, especially cat and dog dander, and wool.
Sometimes the body will even identify a drug meant to be useful as an allergen. For example, many people are allergic to penicillin, so they must have alternative treatment, otherwise their body will respond negatively to penicillin injection. The entire class of sulfonamides, usually called sulfonamides, is another common allergen. Since the consequences of taking these medications if they are allergens can be deadly, most hospitals and doctors check to make sure the patient is not allergic to common medications, and more and more doctors are avoiding these common allergens if it's easy to do so.