An allergic reaction is the body’s way of responding to an invader. When the body senses a foreign substance, called an antigen, the immune system is triggered.

The immune system normally protects the body from harmful agents such as bacteria and toxins.

Its overreaction to a harmless substance (an allergen) is called a hypersensitivity, or allergic, reaction.

Anything Can Be an Allergen

Common dust, pollen, plants, medications, certain foods, insect venoms, viruses, or bacteria are examples of allergens.

Reactions may be in one spot, such as a small skin rash or itchy eyes, or all over, as in a whole body rash. A reaction may include one or several symptoms.

In rare cases, an allergic reaction can be life threatening (see Anaphylaxis). Each year in the United States, over 400 people die from allergic reactions to penicillin, and over 50 people die from allergic reactions to bee and fire ant stings.

Most allergic reactions are much less serious, such as a rash from poison ivy or sneezing from hay fever. The reaction depends on the person but is sometimes unpredictable. Allergies are very common.

Causes of Allergies

Almost anything can trigger an allergic reaction. The body’s immune system has a patrol of white blood cells, which produce antibodies.

When the body is exposed to an antigen, a complex set of reactions begins. The white blood cells produce an antibody specific to that antigen.

This is called sensitization. The job of the antibodies is to detect and destroy substances that cause disease and sickness. In allergic reactions, the antibody is called immunoglobulin E, or IgE.

This antibody promotes production and release of chemicals and hormones called mediators. Histamine is one well-known mediator.

Mediators have effects on local tissue and organs in addition to activating more white blood cell defenders. It is these effects that cause the symptoms of the reaction. If the release of the mediators is sudden or extensive, the allergic reaction may also be sudden and severe.

Your Allergic Reactions Are Unique to You

For example, your body may have learned to be allergic to poison ivy from repeated exposure.

Most people are aware of their particular allergy triggers and reactions. Certain foods, vaccines and medications, latex rubber, aspirin, shellfish, dust, pollen, mold, animal dander, and poison ivy are famous allergens.

Bee stings, fire ant stings, penicillin, and peanuts are known for causing dramatic reactions that can be serious and involve the whole body. Minor injuries, hot or cold temperatures, exercise, or even emotions may be triggers.

Often, the specific allergen cannot be identified unless you have had a similar reaction in the past.

Allergies and the tendency to have allergic reactions run in some families. You may have allergies even if they do not run in your family.

Many people who have one trigger tend to have other triggers as well. People with certain medical conditions are more likely to have allergic reactions.

The look and feel of an allergic reaction depends on the body part involved and the severity of the reaction. Some reactions affect many areas, others affect just one area. Reactions to the same allergen vary by individual.