It mistakenly attacks otherwise harmless substances, such as house dust or certain foods, causing an allergic reaction. Within days to weeks, they form antibodies, or immunoglobulins, a type of protein. The antibodies attach themselves to tissue cells called mast cells, found in the airways, skin, and digestive tract. Later, when pollen is inhaled again, the antibodies react with it, causing the mast cells to release chemicals responsible for inflammation, irritation, and other symptoms of an allergic response. The best known of these is histamine, which can cause sneezing, a runny nose, swelling of the airways, and a skin rash.
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.