Every antibody produced by the immune system is programmed to respond to a specific allergen. For example, an antibody made against cat dander will trigger an allergic reaction only to a cat, not to a dog. Although the antibody IgE is responsible for the majority of allergic reactions, some reactions may involve a type of white blood cell called a T cell, which attacks the allergen directly. Allergic diseases can be acute or chronic, and can range from mild conditions that produce a minor inconvenience to severe disorders that can be life-threatening. Many symptoms of allergy can result from other causes, but when they are triggered by an allergic reaction, they tend to be more severe and chronic.
Allergies usually appear first in childhood and tend to diminish or disappear completely with age.
Doctors don't know why people develop them, but the tendency often runs in families. Types of allergies include those described below. Respiratory allergies . These are the most common allergies, affecting millions of people. Best known is hay fever, or allergic rhinitis, which is characterized by runny nose and eyes, sneezing, and nasal congestion.