The most common hypersensitivity reaction is the allergic reaction. In susceptible people, IgE antibodies are induced when an individual is exposed to such antigens as airborne pollen of grasses, trees, or weeds; animal dander, urine, or saliva; mold spores; various insect-derived dusts and airborne, organic dusts; the venom of stinging in­sects; or certain drugs and foods. Allergens are antigens that produce allergic reactions. As encountered in nature, most allergenic sub­stances contain many different antigens, that is, molecules capable of inducing an immune response. Most of the time, however, only a few of the antigens in these substances act as allergens. In recent years, allergens from a few pollen and animal sources have been identified, characterized, and, in some instances, isolated in pure form. So far, these allergens have proved to be proteins, mostly those in the mo­lecular weight range of 10,000 to 40,000 daltons but which other­wise have no special chemical features. Most commercially available allergenic products used in diagnosis and therapy are simple aqueous extracts of one or more source ma­terials, such as pollen or animal dander. The biologically active com­ponents in these extracts comprise only a small proportion of the total number of components in the extracts. Further, only a few al­lergenic products have been standardized with respect to their active ingredients. Application of the methods of modern biochemistry and immunology to the analysis of allergenic extracts, however, should result in rapid improvement of the quality of the products available to physicians. We do not yet know what it is that leads to the spontaneous pro­duction of large amounts of IgE antibodies in some people. Recent evidence suggests that a person’s total IgE level is genetically deter­mined —perhaps by a single gene. The normal, adult IgE level is less than 750 nanograms (abbreviated ng; 1 nanogram is less than 1 bil­lionth of an ounce) per milliliter (about 350 International units per milliliter), with a mean of 100 to 200 ng/ml (50 to 100 I.u./ml). In allergic individuals, this level is often two to four times above normal, presumably as a result of the individual’s immune response to envi­ronmental allergens. In people infected with certain common parasitic worms, the total serum IgE may be as high as 200,000 ng/ml; usually, however, only a small proportion of this total is parasite-specific IgE antibody. Although the latter finding is thought to indicate that IgE antibodies play a role in immunity to these parasites, there is still no compelling evidence to confirm this hypothesis. As we saw above, exposure to small doses of antigen tends to favor IgE antibody production, production that is regulated by both helper and suppressor T cells. Scientists now know that the level of IgE antibody specific for ragweed-pollen antigens rises dramatically dur­ing and immediately after the annual ragweed-pollen season. The level then falls slowly until the next pollen season, when it again rises. It appears that the annual boost in the production of specific IgE antibody keeps the level of IgE antibody sufficiently high to produce symptoms in individuals exposed to large enough amounts of allergen. Interestingly, even though a pollen-sensitive individual may be ex­posed to allergen for only a few weeks over a year, this person con­tinues to synthesize enough IgE antibody to maintain detectable serum and tissue levels throughout the year. It has been suggested, but not proved, that this may result from a disturbance among regu­latory T cells in people who produce enough IgE to make them allergic.

Cytotoxic food tests are tests in which a per­son’s white cells are used to diagnose an allergy. In the test, the white cells are incubated for a given period with an extract of the suspect food or inhalant. After incubation, the white cells are examined to determine whether any changes have occurred. Numerous technical difficulties are involved in administering this test. In addition, reports in which the test has been subjected to rigid experimental study have failed to demonstrate the validity of this type of test.

The word allergy, from the Greek alios ("other") and ergeon ("action"), was coined in 1906 by Baron Clemens von Pirquet, who recognized that the introduction of a foreign substance into a tissue can alter the tissue’s capacity to react to a subsequent encounter with the same substance. Von Pirquet discovered that this altered response is both protective (that is, it is an immune response) and potentially harmful (that is, it can cause a person to become hypersensitive). Since von Pirquet’s time, physicians and researchers have redefined the term, and it is today synonymous with hypersensitivity. Many allergists prefer the restricted meaning: "hypersensitivity reactions caused by allergic antibodies."

An antihistamine is a drug used to neutralize or in­hibit the action of histamine; antihistamines are divided into six classes in the treatment of allergic disorders: ethanolamines, ethylene-diamines, alkylamines, piperazines, and phenothiazines; the sixth class is a group of miscellaneous compounds.

Nasal polyps are round, small, smooth, soft masses— single or multiple—that form in the sinus or nasal cavities. They can occur in patients with severe allergies, or they may appear as a result of chronic inflammation in patients with vasomotor rhinitis. In some patients, polyps occur without a discernible cause. Removal of the polyps by surgery may be necessary, especially when they ob­struct the nasal passages; but in some cases, polyps can be treated with cortisone.

In medicine, adjuvant is a substance that, when added to an antigen, enhances the production of antibodies.

Carotid bodies are small organs present in the neck. They have a protective function, which is to respond to rapid drops in the oxygen level and to increases in the carbon dioxide level. Re­section of the carotid bodies, called glomectomy, was once promoted as treatment for asthma; controlled studies, however, did not show significant improvement after surgery, and the treatment has been abandoned.

Intrinsic asthma is a nonseasonal form of asthma caused by respiratory infection or by other precipitating factors, not by allergies.

In some cases, yes. Severe hay fever, if left untreated, can lead to nasal polyps and sinusitis. Patients with allergic rhinitis who also have nasal polyps should undergo allergic evaluation and therapy to help prevent further growth of the polyps. Allergic dermatitis, or eczema, that is not treated early can spread and occasionally be complicated by secondary infection. The patient who originally has only occasional periods of asthma may develop a chronic condition. If the allergies are detected early, however, and appropriate treatment is begun, the condition may improve or at least be controlled.

Ragweed, a plant belonging to the family Compositae, is the major cause of hay fever in the United States. The ragweed (Compositae) family is large, with approximately 15,000 species. Although the number of wind-pollinated species is relatively low, they produce and release a tremendous amount of pollen. Members of the ragweed genus Ambrosia are plentiful over much of North America, growing as weeds in vacant lots, on farmland, and in cities and along the road. Ragweed plants are short-day plants that are brought into flower by the waning days of late summer. Ragweed-induced hay fever begins earlier in the northern latitudes than in the southern latitudes and usually extends to October. Because of the amount of agricultural land and the pattern of air movement across North America, few areas of the United States are free of ragweed. The areas with little ragweed are those forested with northern coni­fers, southern Florida, and most of the Southwest and Pacific coast.