What happens when people with allergies encounter an allergen?

When people with allergies encounter an allergen, their immune system starts making antibodies - specifically IgE antibodies. These antibodies are specific to the allergen, like cat dander, ragweed or dust.

Thousands of these antibodies bind with the surface of special cells in body tissue known as mast cells, which lie in wait for your next exposure to the allergen in question. While they wait, these mast cells absorb a number of different chemicals from the blood to aid in the body's defense. They store these chemicals in very tiny granules. When you're re-exposed, the offending allergen binds to the IgE antibodies that are bound to the mast cells, making the mast cells release the chemicals.

One of these chemicals, called histamine, is probably quite familiar to you. It's one of the key players in the allergic response system and causes a great number of the reactions, including runny nose, itchy and watery eyes, and sneezing. Most anti-allergy medications treat you by blocking histamine from binding to its receptor; these medications are called antihistamines.

The allergic reaction can have an early and a late phase. Generally, the early phase may start within a few minutes of exposure. The late phase, on the other hand, may start several hours after initial exposure. The early phase gets caused by the release of the chemicals stored in the mast cells' granules. The late phase is caused by other various inflammatory cells recruited into the area.