What causes allergy symptoms?

If you are exposed to a substance you're allergic to, your immune system will send out an all-points bulletin, causing your body to produce proteins called immunoglobulin E (IgE) antibodies. Their job is to fight off any foreign invader that threatens the body. During this process, your body releases histamines, chemicals that cause allergy symptoms. That's why some drugs that relieve allergy symptoms are called "antihistamines."

An allergy is an overreaction by the immune system to some type of allergen that the body is exposed to.

When that invader enters the body, often through the nose, the body tries to protect itself by producing those antibodies called immunoglobulin E (IgE). That sets off a chain reaction in which IgE sends a signal to allergy cells (mast cells) to release chemicals that fight the allergen. When those chemicals (some are called histamines; others belong to the cytokine system) are released into the bloodstream, they produce inflammatory reactions throughout the body, not just at the site of invasion. It's those reactions that trigger the itchy eyes, the runny nose, the hives, etc.