Allergy

An allergy is an abnormal, acquired sensitivity to a given substance, including pollen, drugs, or numerous environmental triggers.

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 can arise if inflammation swells tissue surrounding the tiny sinus drains.