Allergies; DEFINITION Allergies occur when your immune system reacts to a foreign substance — such as pollen, bee venom or pet dander — that doesn't cause a reaction in most people.

Your immune system produces substances known as antibodies. Some antibodies protect you from unwanted invaders that could make you sick or cause infection.

When you have allergies, your immune system makes antibodies that identify a particular allergen as harmful, even though it isn't. When you come into contact with the allergen, your immune system's reaction can inflame your skin, sinuses, airways or digestive system.

The severity of allergies varies from person to person and can range from minor irritation to anaphylaxis — a potentially life-threatening emergency. While most allergies can't be cured, a number of treatments can help relieve your allergy symptoms.

SYMPTOMS Allergy symptoms depend on the substance involved and can involve the airways, sinuses and nasal passages, skin, and digestive system. Allergic reactions can range from mild to severe. In some severe cases, allergies can trigger a life-threatening reaction known as anaphylaxis.

Hay fever, also called allergic rhinitis, may cause:

Sneezing Itching of the nose, eyes or roof of the mouth Runny, stuffy nose Watery, red or swollen eyes (conjunctivitis