Allergy reactions is sometimes a disorder of the immune system and often called an atopy. Most allergy reactions occur by way of normally harmless environmental substances known as allergens; these reactions are atained, predictable, and fast. For the most part, an allergy is one of four types of hypersensitivity and is called type I (or immediate) hypersensitivity. It is characterized by excessive activation of certain white blood cells called mast cells and basophils by a type of antibody known as IgE. These result in an intense inflammatory response. Common allergic reactions include eczema, hives, hay fever, asthma, food allergies, and reactions to the venom of stinging insects such as wasps and bees.

There are now a wide variety of tests that exist to diagnose all types of allergic conditions; these include testing the skin for responses to known allergens or analyzing the blood for the presence and levels of allergen-specific. Treatments for allergies include allergen avoidance, use of anti-histamines, steroids or other oral medications, immunotherapy to desensitize the response to allergen, and targeted therapy. Many mild allergy reactions such as hay fever are highly prevalent in the human population and cause symptoms such as allergic conjunctivitis, itchiness, and runny nose. Allergies do play a large part in conditions such as asthma. In some people, severe allergies are caused by environmental or dietary allergens or medications. These types of allergy reactions may result in life-threatening anaphylactic reactions and potentially death.

Certain substances that come into contact with the skin, like latex, are also common causes of allergy reactions; these are known as contact dermatitis or eczema. Skin allergy reactions often cause rashes, or swelling and inflammation within the skin, in what is known as a "wheal and flare" reaction classified as hives and angioedema. If the skin rash that forms as a result of an allergy reaction develops blistering, then this is a sure sign of a more serious allergic reaction, and one would be best advised to seek further medical help.

According to the hygiene hypothesis, which was introduced by David P. Strachan, allergy diseases are caused by incorect immune system responses to harmless antigens driven by a TH2-mediated immune response. Many bacteria and viruses elicit a TH1-mediated immune response, which down-regulates TH2 responses. One of the first introduced mechanism of treatment of this hygiene hypothesis said that not enough stimulation of the TH1 arm of the immune system lead to an overactive TH2 arm, which in turn led to allergic disease. In other words, Some people living in too sterile an environment are not exposed to enough pathogens to keep the immune system busy. Our bodies evolved to deal with a certain level of such pathogens, when it is not exposed to this level the immune system will attack harmless antigens, and thus normally benign microbial objects, like pollen, will trigger an immune response.