The form of treatment known as immunotherapy is also called desensitization, hyposensitization, or allergy shots. Im­munotherapy is used in the treatment of allergic patients with respiratory symptoms, mainly of hay fever or asthma. In this form of treatment, injections of allergenic extract are given in gradually in­creasing amounts of allergenic extract over a period of months. The goal is to induce a degree of tolerance to the allergens and to bring about a decline in the symptoms and the medication requirements. The mechanism involved is not entirely clear. Among the changes that occur in patients after immunotherapy are the development of blocking IgG antibodies (protective antibodies), a gradual decline in the level of allergic (IgE) antibodies against the offending sub­stance, and a decrease in the patient’s cell sensitivity for the release of histamine upon exposure to the allergen. Although no uniform schedule or dosage has been established for administering allergenic extracts, strong scientific evidence exists to support the belief that greater amounts of extract administered to a patient produce more satisfactory results than do smaller doses. The administration of very small doses of allergenic extract is popular among some specialists, but such treatment is not supported by adequate experimental studies. Currently, the main use of immunotherapy is in treating patients who are sensitive to inhaled allergens, mainly pollen, mold, and house dust. The administration of food extracts to patients highly sensitive to foods is contraindicated because of the chance of a severe anaphy­lactic reaction. Immunotherapy involving food extracts in patients with low sensitivity to food allergens has not been demonstrated as valid in well-designed clinical studies.

Antibodies are substances produced in response to a foreign substance (antigen) that are capable of reacting with that particular antigen. Antibodies are contained in a special type of pro tein called an immunoglobulin, which is produced by specialized "plasma" cells. Antibody activity in the serum is confined to a hetero­geneous group of gamma globulin molecules. Antibodies belong to one of five immunoglobulin classes: (1) immunoglobulin G (IgG); (2) immunoglobulin A (IgA); (3) immunoglobulin M (IgM); (4) immunoglobulin D (IgD); or (5) immunoglobulin E (IgE). They are an important part of the defense, or immune, mechanism, which, in turn, is essential to survival. Under certain conditions, antibodies may cause allergic tissue damage, called hypersensitivity reaction {see also hypersensitivity). The antibodies responsible for hypersensitivity re­actions of the allergic type, such as hay fever and asthma, belong to the IgE class. See also Immunoglobulins. Antibodies, Blocking A blocking antibody is an antibody of one class that combines with an antigen, thus preventing the antigen from re­acting with an antibody of another class. Anticholinergic See Bronchodilator. Antigens Antigens are foreign substances that are capable of induc­ing an immune response in normal individuals. Most of the naturally occurring antigens are large molecules and, more frequently, pro­teins, although high-molecular-weight polysaccharides (sugars) are also potent antigens. An immune response consists of a humoral re­sponse mediated by antibodies and a cellular immune response medi­ated by mononuclear cells. See also Allergens.

Alveoli are the tiny air sacs in the lungs.

All of the following drugs dilate the air passages (the bronchi and bronchioles) of the lungs as well as affecting, in numerous ways, the blood vessels, heart, small intestines, central nervous system, and, in women, the uterus: epinephrine aqueous suspension (Asmolin and Sus-Phrine); epinephrine hydrochloride (Adrenalin Chloride in a 1:1,000 solution for subcutaneous or intra­muscular injection; available in 1:100 solution for oral inhalation under many trade names); ephedrine (Ephedrine USP); and ethyl-norepinephrine (Bronkephrine). Epinephrine hydrochloride is often the first drug used in treating allergic or allergylike reactions involv­ing multiple organs. Ephedrine is available for both injection and oral use, but most often it is used orally in one of many fixed-ratio combi­nations of drugs. Methoxyphenamine, which is similar to ephedrine, is taken orally. Isoproterenol (available under many trade names) and protokylol (Ventaire) both dilate the air passages of the lungs and affect the blood vessels, heart, small intestines, and, in women, the uterus. Iso­proterenol is available for oral inhalation, as a lozenge to be placed under the tongue, and as a 1:5,000 solution for subcutaneous injec­tion. Protokylol is taken orally. Metaproterenol (Alupent and Metaprel), terbutaline (Brethine and Bricanyl), and salbutamol (Albuterol and Proven til) are rela­tively selective in dilating the airways of the lung and affecting the blood vessels and the uterus. Metaproterenol has been licensed for use orally or by inhalation. Terbutaline is now available as both an oral and an injectable preparation, and it may soon appear in inhala tion form. Salbutamol is available in Europe and some other areas in metered-dose nebulizers for inhalation, but it has not been licensed in the United States.

A bronchodilator is medication that causes bron-chodilation (relaxation of the bronchial muscles). It is administered orally, intravenously, by inhalation, or by injection.

Irritation of the stomach or intestines, the most common side effects of methylxanthines, can lead to nausea and some­times to abdominal cramping. Headache and vomiting are associated with excessive doses. Because these drugs stimulate secretion of stom­ach acid, patients with ulcers of the stomach or intestines should be cautious about using them. Methylxanthines can also affect muscles, which prevent the contents of the stomach from returning to the esophagus through a valvelike mechanism known as the gastroesoph­ageal sphincter, or the cardiac sphincter. Relaxation of this sphincter permits material to flow backward from the stomach into the esoph­agus. The use of aminophylline in the form of rectal suppositories re­sults in unpredictable aminophylline blood levels; consequently, their effect in treatment becomes unpredictable. The prolonged use of suppositories can even cause rectal irritation and bleeding. If symp­toms of central nervous system stimulation- —such as irritability, rest­lessness, or insomnia—occur, the dose should be reduced. Muscle tremor, or shakiness, may also occur. Overdosage can cause nervous agitation, heightened reflexes, fever, vomiting of blood, convulsions, abnormal heart rhythm, and even death.

Blood groups are categories of blood determined by inherited antigens present on red blood cell surfaces. The most im­portant blood groups are А , В , АВ , and O; it is these groups that are referred to in speaking of "incompatible" blood.