In some cases, an individual’s immune system becomes sensitized to a substance that is actually not a threat. In allergies, the immune system has incorrectly identified a substance (called an allergen) as dangerous. As a result, the immune system attacks the allergen and triggers an allergic cascade.

Common allergens include pollen, dander, mold, dust, foods, and other substances. An allergic reaction to these substances produces symptoms similar to those felt when the immune system attacks an actual threat, such as a bacteria or virus. These symptoms may include sneezing, runny nose, coughing, itching, nausea, diarrhea or shortness of breath.

The first time a person ever encounters an allergen, they become sensitized and do not actually have an allergic reaction. Sensitization is the process through which the immune system is able to classify an allergen as potentially dangerous, and remember it in case of future contact. Once a person has been sensitized to an allergen, they will have an allergic response the next time (and any other time) that allergen is encountered.

With an allergic response, the immune system reacts in much the same way it would if a dangerous antigen were present. The chain of events that the immune system follows includes:

1. The allergen to which a person is sensitized enters the body. It may be inhaled through nasal passages, come in direct contact with the skin or be ingested. The allergen flows through the bloodstream and encounters immunoglobin E (IgE) antibodies.
   
   
2. The IgE antibodies recognize the invader and bind themselves to the allergen molecule. They also trigger the mast cells and basophils to which they are attached to self–destruct.
   
   
3. Powerful chemicals from inside the mast cells and basophils are released into the bloodstream. These include histamines, leukotrienes and other allergy stimulators.
   
   
4. Allergy symptoms begin to appear. These may be localized (only in the area where these chemicals were first released) or systemic (throughout the entire body). These chemicals mainly affect the blood vessels, mucous glands and bronchial tubes.

The allergens are therefore the match that lights the fuse (IgE) that triggers the bomb (mast cells and basophils) to explode.

Sometimes one substance is similar enough to another that the immune system will mistake it for a known allergen and trigger the allergic cascade by mistake. For example, a protein in latex is similar to a protein found in fresh fruits, vegetables and nuts. People with an allergy to latex often have allergic reactions to these foods. This phenomenon is known as cross-reactivity.

A number of medications are used in the treatment of allergies. Some of these work to prevent or relieve symptoms by interrupting the immune system’s response to allergens. For instance, antihistamines and mast cell stabilizers prevent the release of, or block the effects of, histamines. Leukotriene modifiers are another type of allergy drug which work by blocking the effects of leukotrienes.

Immunotherapy is another form of allergy treatment. Also known as allergy shots, this method involves injecting a patient with increasing, controlled doses of an allergen over a period of time. This allows the immune system to slowly build immunity to the allergen, and as a result, it no longer reacts as strongly when the body comes in contact with the substance.