Most humans mount significant IgE (immunoglobulin E (IgE) is a class of antibody) responses only as a defense against parasitic infections. However, some individuals mount an IgE response against common environmental antigens. This hereditory predisposition is called atopy. In atopic individuals, non-parasitic antigens stimulate inappropriate IgE production, leading to type I hypersensitivity. A nonparasitic antigen capable of stimulating a type I hypersensitivity reaction in atopic individuals is called an allergen.

Sensitivities vary from one person to another and it is possible to be allergic to an extraordinary range of substances.

Dust, pollen and pet dander are all common allergens, but it is possible to be allergic to anything from chlorine to perfume. Food allergies are not as common as food sensitivity, but some foods such as peanuts (really a legume), nuts, seafood and shellfish are the cause of serious allergies in many people.

Officially, the Food and Drug Administration does recognize 8 foods as being common for allergic reactions in a large segment of the sensitive population, which includes, peanuts, tree nuts, eggs, milk, shellfish, fish, wheat and their derivatives, soy and their derivatives, and sulphites (chemical based, often found in flavors and colors in foods) at 10ppm and over.  It should be noted that other countries, due to differences in genetic profiles of its citizens and different levels of exposure to different foods, the "official" allergen list will change. Canada recognizes all eight of the allergens recognized by the US, and also recognizes Sesame Seeds.

Poison ivy is a plant that will cause an allergic reaction in 70-85% of humans. But, given enough repeated contact—like any allergy, most human bodies will learn to fight the allergen.

An allergic reaction can be caused by any form of direct contact with the allergen—eating or drinking a food you are sensitive to (ingestion), breathing in pollen, perfume or pet dander (inhalation), or brushing your body against an allergy-causing plant (direct contact, generally resulting in hives). Other common causes of serious allergy are wasp, fire ant and bee stings, penicillin, and latex. An extremely serious form of an allergic reaction, which can kill in mere minutes, is called anaphylaxis. One form of treatment is the administration of sterile epinephrine (via "Epi-Pen") to the person experiencing anaphylaxis, which suppresses the body's overreaction to the food ingested, and allows for time to be transported to a medical facilty (it does not "cure" the allergic reaction).