The human body's overly sensitive reaction to certain foreign but usually harmless substances is known as an allergy. Some people suffer from hay fever when pollen is in the air. Others develop skin rashes when they touch certain substances. Still others experience stomach cramps after eating particular foods. When anyone reacts in these ways to any outside substance, that individual is said to be allergic to the substance. Not everyone is allergic to the same things. A substance that triggers an allergic reaction in one person may be harmless for another person.

Allergens and the immune system

Vertebrates, or animals with backbones, have natural defenses against diseases. These defenses are known as the immune system. The human immune system is the most advanced. Any outside substance that provokes a response from the immune system is called an antigen. Antigens that cause allergies are called allergens. The most common of these include dust, pollens, foods, animal danders (bits of dry skin or fur), insect venoms, cosmetics, soap, and drugs.

The immune system has many ways of protecting the body against allergens. Specific cells of the immune system, called lymphocytes, have the ability to react to specific allergens. Lymphocytes are blood cells that help the body react to many different foreign substances.

Certain lymphocytes, called B cells, produce antibodies. Antibodies can destroy, or neutralize, allergens. The antibodies that are produced to fight specific allergens can react only to those allergens. They will not respond to allergens in other substances. For instance, if a person allergic to shrimp eats it, shrimp allergens will provoke the formation of antibodies whose only job is to work against shrimp allergens and not, say, pollen allergens. (See also immune system.)

Causes and development of allergies

Many people with allergies belong to families in which parents and other close relatives also have allergies. An inherited tendency to asthma, for instance, appears to be quite common. Specific allergies, such as those to pollen, are not inherited. A person becomes sensitive, or allergic, to pollen as a result of exposure to it.

Regardless of the different causes of allergies, the process by which they affect the body can be described in a general way. Several things happen to cause an allergic reaction. First, a vulnerable person is exposed to a substance such as pollen. The bodies of most people would simply ignore pollen, but some bodies treat it as a foreign invader. When this happens, the immune system produces antibodies to fight the pollen allergens.

The antibodies produced in the body as a result of the first fight with the allergen tend to stay in the blood. The person then becomes sensitive to the allergen. The immune system remembers and recognizes the specific allergens that attacked the body. As a result, even a brief repeated exposure to the allergen can result in an allergic reaction. The next time the body comes in contact with the substance, the antibodies react and release a chemical called histamine from cell tissues. This is the most important factor in producing symptoms of allergy.

Histamine is found in most animal and some vegetable tissues. It is neutral, or harmless, while it remains inside the cells. When histamine is released, fluid and cells of the immune system leak through the walls of the blood vessel and go to the site of injury or infection, where they begin to fight the infection. This leakage of fluid causes the tissues to swell, a condition called edema. That is why the eyelids of a person with hay fever become red and puffy. Histamine also causes certain muscles, called smooth muscles, to contract. Smooth muscles are found mostly in the internal organs, the intestines, blood vessels, and breathing passages. The effect of histamine on these muscles can cause wheezing or cramps and pain in the stomach.

Anaphylactic shock, or anaphylaxis, is an exceptionally severe type of allergic reaction. Almost any substance can cause anaphylaxis, but the most common agents are drugs such as penicillin, foods such as nuts and shellfish, and insect venom. Symptoms of anaphylaxis include itching of the scalp and tongue, difficulty in breathing, skin flush of the whole body, abrupt fall in blood pressure, vomiting or stomach cramps, and unconsciousness. Anaphylaxis sometimes leads to death.

Treatment

Most people quickly learn which allergies they have. When possible, they try to avoid contact with the allergen. Many substances, such as dust, pollen, and polluted air, are so widespread, however, that it is difficult to avoid them. Taking drugs known as antihistamines can decrease reactions to these allergens. Antihistamines prevent histamine from reaching certain vulnerable tissues and so prevent the allergic reaction.

For certain allergies, it is possible to make a person less sensitive to the offending allergen. This is done by a specialist who injects a small amount of the allergen under the skin. The dose is gradually increased until the body no longer views the allergens as a foreign substance to be attacked. The allergic reaction is thus eliminated.