* An allergy is an abnormal, acquired sensitivity to a given substance, including pollen, drugs, or numerous environmental triggers. The term was coined by the Viennese pediatrician Clemens von Pirquet in 1906 after noting that some of his patients were hypersensitive to normally innocuous entities such as dust, pollen, or certain foods. Pirquet called this phenomenon "allergy", from the Greek words allos meaning "other" and ergon meaning "work".

** Signs and symptoms
Allergy is a local or systemic inflammatory response to allergens. Local symptoms are:

* Nose: swelling of the nasal mucosa (allergic rhinitis)
* Eyes: redness and itching of the conjunctiva (allergic conjunctivitis)
* Airways: Sneezing, bronchoconstriction, wheezing and dyspnea, sometimes outright attacks of asthma, in severe cases the airway constricts due to swelling known as anaphylaxis.
* Ears: feeling of fullness, possibly pain, and impaired hearing due to the lack of eustachian tube drainage.
* Skin: various rashes, such as eczema, hives (urticaria) and contact dermatitis.
* A headache from sinus pressure[6] can arise if inflammation swells tissue surrounding the tiny sinus drains.
Allergic rhinitis afflicts 20% of the US population. Also known as hayfever, symptoms can be in response to airborne pollen. Asthmatics are often allergic to dust mites. Apart from ambient allergens, allergic reactions can result from foods, insect stings and reactions to medications.

Systemic allergic response is also called anaphylaxis; multiple systems can be affected including the digestive system, the respiratory system, and the circulatory system. Depending of the rate of severity, it can cause cutaneous reactions, bronchoconstriction, edema, hypotension, coma and even death. This type of reaction can be triggered suddenly or the onset can be delayed. The severity of this type of allergic response often requires injections of epinephrine, sometimes through a device known as the Epi-Pen auto-injector. The nature of anaphylaxis is such that the reaction can seemingly be subsiding, but may recur throughout a prolonged period of time.

*** Diagnosis

- skin testing
- Considerations with skin test
- Blood testing

**** Treatment
- Nasal Irrigation
- Increasing use of chemicals
- The "hygiene hypothesis"