To better understand what goes on during a typical allergy attack, you first need to know a few basic facts about your immune system, which is responsible for allergies. In healthy people the immune system is the body's natural defense network against invasion by parasites, fungi, bacteria, and viruses. In addition, it functions to root out and destroy other unwanted "parasites," such as malignancies, in their very early stages, before they have a chance to spread. In fact, without a properly functioning immune system, we would all surely die from a host of infections and cancers. It is precisely for this reason that victims of AIDS, who suffer from severely compromised immune function, succumb to various types of infections and malignant tumors that do not ordinarily affect healthy (that is, immunologically normal) individuals.

Although a listing of only the most common allergens in our environment would fill many books, there are just three main routes for them to gain access to the body. They may be inhaled, swallowed, or absorbed through the skin and mucous membranes. This information is important since it is the route the allergens take that determines the symptoms a person suffers. Pollens, house dust, and chemical fumes, for example, are inhaled and therefore trigger the stuffy head, runny nose, and sneezing of hay fever and other respiratory allergies. Foods and drugs taken by mouth can give rise to local gastrointestinal symptoms, and once absorbed through the intestines into the bloodstream, they are capable of causing widespread allergic reactions such as hives. Following direct contact with the skin, certain plants, such as the all-too-familiar poison ivy, typically precipitate an intensely itchy, blistering, and oozing eruption. Finally, by injecting toxins or other substances into the skin (which eventually reach the bloodstream), a multitude of stinging or biting insects may provoke local hive like reactions or severe systemic allergic symptoms, such as respiratory difficulty, hypertension, and shock.

Regardless of where allergies take place in the body or what the specific symptoms are, allergic reactions result from the reaction to the substance by one or more of the three major components of the immune system: (1) cells known as lymphocytes, plasma cells, and mast cells, (2) particular types of proteins called antibodies, and (3) a variety of chemical substances known as mediators.