There are almost as many types of allergies as there are allergy sufferers. They are usually classified according to what causes them, or the symptoms they cause. Allergens may cause a reaction in several ways: inhalation, injection, ingestion, or through skin contact. Allergic reactions can involve any part of the body but most frequently affect the nose, eyes, lungs, and skin.

Allergies that are defined by what causes them:

• Inhalant allergy, such as from pollen or dust
• Infectious allergy, with symptoms made worse by a cold or flu
• Insect allergy, usually from the bite of a particular insect
• Drug allergy, which can be quite serious and may result in anaphylaxis—a life-threatening condition
• Physical agent allergy, such as an allergy to cold, heat, or exercise
• Contact allergy, such as to latex, household chemicals, or newsprint
• Food allergy, including anything that you could ingest that is not a poison; it is different from a food sensitivity. Food allergies are generally severe and will cause a noticeable reaction

Allergies that are defined by their symptoms:

• Allergic rhinitis or hay fever, the most common form of allergy
• Eczema
• Hives, also known as chronic urticaria
• Skin rashes, including rashes that are not included in any other grouping
• Rosacea
• Anaphylactic shock