Although medical controversy focuses on the causes and mechanisms of allergy even on the definition of the term to the lay person the chief mystery is, how can a person suddenly become allergic to things that previously caused no harm? And why do certain things affect one person strongly and others not at all?

Much remains to be learned. But from the beginning, the word “allergy” has been used to describe a type of adverse reaction by the body’s immune system to a foreign substance. Normally, the immune system develops antibodies only as a response to the presence of a potentially harmful substance (such as a virus). The antibodies produced are then specific to that substance, and will be produced again immediately if that substance is encountered later. In an allergy, this mechanism is brought into operation by a harmless substance (like pollen or some foodstuffs). Re-exposure to this substance known as an allergen results in the production of antibodies, and a subsequent allergic reaction.

Food allergies in babies may be one cause of hyperactivity, in which the baby cries and moves incessantly. Among the chief culprits are food colorants added to some foods (such as orange juice), although even milk causes an allergic reaction in some babies.

Allergic reactions

Many allergic reactions involve a common mechanism the over-production by the body of a type of antibody known as IgE. This antibody binds to the surface of a special cell called a mast cell. When the IgE antibody combines with the allergen, the mast cell releases its contents, called H-substances. These include histamine, prostaglandins, and leukotrienes, which are the most important mediators of allergic reactions. These mediators cause tissue swelling, because they increase local blood flow and also cause small blood vessels to leak fluid into the surrounding body tissues. Several common symptoms of allergy result, for example, in swollen, red, itching lumps in the skin (hives), a stuffed-up, runny nose (rhinitis), or red, itching eyes (conjunctivitis).
Some allergic reactions are relatively minor, but mast cell allergic mediators also cause more severe reactions, such as anaphylaxis or asthmatic attacks. Anaphylaxis occurs when histamine is released into the bloodstream in large quantities. This produces a sudden fall in blood pressure, which causes physiological shock. There also may be intense swelling of tissues in the throat, which can cause asphyxiation. Anaphylaxis is rare, but can occur if a susceptible person is injected with a drug, such as penicillin, to which he or she is allergic, or suffers an allergic reaction to insect stings, in an asthmatic attack, the release of histamine and leukotrienes in the walls of bronchi in the respiratory tract causes the smooth muscle to contract, thus narrowing the bronchi and causing partial respiratory obstruction. The characteristic asthmatic wheezing and difficulty in exhaling results. Another common cause of allergic reaction is certain food, and recent studies suggest that migraine headaches may be associated with food allergies in some way.

Certain allergies, especially hay fever, asthma, and eczema, often run in families. This pattern is known as atopy, and those who are affected are described as atopic. Individual members of a family may be afflicted with one or all types of reaction, due to excess IgE production. The exact reason for this is not known, but it is probably due to a minor abnormality in the complex control systems that regulate immune responses to foreign substances. Because the immunoregulatory network changes during an individual’s lifetime, new allergies may appear and old ones disappear in a totally unpredictable manner.

Allergy and the twentieth century

Current thinking suggests that the changes in diet, in the environment, and in the stresses of everyday life characteristic of modern times actually encourage the development of allergies, both by increasing the number of powerful potential allergens and by increasing the overall susceptibility of people.

Chemical fertilization, allowing the land to be overexploited, reduces the amount of trace substances in the harvested products. Refining common ingredients such as flour and sugar also reduces the level of trace elements that once probably afforded some protection against the development of various sensitivities. In addition, the use of chemical additives, especially colorants, in food has greatly increased.

A further dietary factor seems to be the contemporary fashion for early bottle-feeding and weaning of infants. A baby’s own immune system is not fully developed until about nine months of age, and allergies from which maternal antibodies in milk may afford some protection can develop easily before this time.

Our natural environment is now heavily polluted. Many of the pollutants either bring on allergic states or are themselves potential allergens. They include many chemicals, such as gasoline, cosmetics, and cleaning products.

Another possible reason for the increasing incidence of allergies is that, with growing public awareness, it is becoming fashionable to diagnose them as a cause of illness.

Allergy diagnosis

If inhalation, contact, or ingestion of a substance results in an allergic reaction with obvious symptoms, diagnosis of allergy is easy. An example is a rash that develops after eating strawberries. But allergic symptoms may also derive from a cause that is less easily distin guished. The oldest method of tracking down the source of such “hidden” allergies is the exclusion (or elimination) diet, in which a simple, basic diet that produces no reaction is gradually enlarged over several weeks by adding suspect foods one after another until the reaction occurs. A recent study involving children with severe migraine has shown that marked improvements can be achieved by excluding certain dietary elements.

More modern means of diagnosis are skin tests and blood tests. Skin tests are of two types. Prick or scratch tests are suitable for some types of allergy; patch tests are better for other sorts of allergies. Blood tests are more accurate; the most common are the radio-allergosorbenttest (RAST) and the cytotoxic tests.

There are thousands of potential allergens, and the chief types are shown in the diagram: animal fur and dander, feathers, mites in house dust, pollen, foods such as milk, eggs, strawberries, and shellfish, alcoholic drinks, drugs, cosmetics, soaps and shampoos, atmospheric pollutants, and man-made fibers.

Allergy treatment

Therapies range from the orthodox to the unusual. Some relieve the symptoms rather than actually treating the underlying allergy. This is particularly true of the suppressive drugs commonly prescribed, such as sodium chromoglycate, antihistamines, and steroid preparations.

The method known as desensitization attempts to reduce the allergic response gradually. This treatment starts with the administration of a very dilute form of the allergen, then slowly increases the amount of allergen that can be tolerated without producing symptoms.

Another treatment is to avoid the allergen. Appropriate food can be excluded, or ionizers can be used to precipitate pollens out of the air, for example. And, given time, many allergies or sensitivities disappear of their own accord.

Finally, some people are apparently allergic to everything in their environment a condition sometimes described as “total allergy syndrome.” It is possible that they have a breakdown in their immune system that causes the easy development and endless accumulation of allergies. It is more probable, however, that their condition has been falsely blamed on allergies and is, in fact, something else that has yet to be properly understood.

Once an allergic reaction begins, the results can be rapid and dramatic—particularly on the skin. The woman in this photograph is allergic to adhesive tape, and contact with only a small area caused an eruption of lesions over nearly all her body.