The term allergy (derived from antigen, a substance capable of giv­ing rise to the production of antibody) was coined by Baron Clemens von Pirquet in 1906 to indicate a state of changed reaction to a for­eign substance (which von Pirquet called an allergen). Allergens, von Pirquet said, differ from antigens, in that they can lead not to the production of antibody but to a condition of hypersensitivity. In his view, the term immunity should be restricted to those processes in which the introduction of a foreign substance into an individual does not cause a reaction that can be detected with medical tests; in such cases, individuals would be in a state of complete insensitivity. On that basis, it is apparent that, to understand and control allergic diseases, we must understand the immune system. In this century we have seen a remarkable expansion of our knowledge of the immune system, knowledge that has been used to gain insight into the relation­ship of the immune system to allergic diseases. Further advances in the ability to diagnose and treat allergic diseases depends largely on the application of this knowledge to the treatment of allergic people. Author’s Note Because the field of immunology is so broad, this chapter could only highlight a few of its salient features. For a comprehensive treat­ment of the field of immunology, the interested reader is referred to : Samter, Immunological Diseases, 2nd ed. (Boston: Little, Brown, 1971); Middleton et al., Allergy Principles and Practice (Saint Louis: C. V. Mosby, 1978); and Fudenberg et al., Basic and Clinical Immu­nology (Los Altos, Cal.: Lange Medical Publications, 1980).

Drugs commonly used to treat allergic diseases act either by sup­pressing the release of mediators from mast cells and basophils (diso-dium cromoglycate acts in this way) or by antagonizing the effect of the mediators on tissue targets (antihistamines). The action mecha­nism of corticosteroids such as prednisone is not known. Recent evidence suggests that corticosteroids may reduce the number of mast cells and basophils in the target tissues. As more is learned about allergic reactions and how mediators act, improved drags can be developed. The simplest, most straightforward way to relieve the symptoms of allergic diseases is to avoid the allergens that cause them. This is done fairly easily when the source of the allergen can be recognized and avoided, as, for example, in the case of a pet cat. When the source is as widespread as ragweed pollen, however, avoidance is not practical. In this century, extracts containing biologically active components of allergens have been used extensively in treating allergic diseases. At first, immunotherapy, the regular, systematic administration of these extracts, was intended to immunize people against the supposed toxic effect of allergens. As the role of hypersensitivity in allergic diseases became known, the concept of hyposensitization was intro­duced, in which successful immunotherapy was thought to result in a reduced level of sensitivity. But it did not result in a complete loss of sensitivity. Later it was shown that immunotherapy induces a new type of antibody, called a blocking antibody. The use of modern immunologic methods has proved that the blocking antibody is an IgG antibody capable of reacting with allergen in the body fluids before it reaches the mast cells and basophils in target organs. Ele­vation of allergen-specific IgG antibody does indeed occur during immunotherapy; this seems important in the successful treatment of allergy to stinging insects. It has also been found that immunotherapy blunts the seasonal booster effect of inhaled allergen on IgE antibody. This effect is followed by a gradual decline in the level of the par­ticular antibody. What is not yet clear is whether these changes ac­count for the relief of symptoms that occurs with successful immuno­therapy for allergy to such inhaled substances as pollen. Another clue to the mechanism of immunotherapy is the recent observation that immunotherapy is accompanied by an increase in suppressor T cells. It seems reasonable to assume that these T cells are related to the changes described above. Furthermore, it is possible that the T cells are also involved in regulating the flow of basophils and mast cells to target organs. If such is the case, perhaps another effect of immunotherapy will be a reduction in the number of sensi­tized cells present in the target tissues of an individual being treated. Further research in the immunologic aspects of allergic diseases should provide a better understanding of their mechanisms and should, as well, put immunotherapy on a more rational basis.

Antibodies belong to a group of proteins called immunoglobulins (abbreviated Ig) found in serum and other body fluids. An antibody is an immunoglobulin that reacts with a given antigen. For example, an Ig that binds ragweed pollen antigen is an antiragweed-pollen antibody. To understand how antibodies function, we must first un­derstand their structure. Like all proteins, immunoglobulins are made up of smaller units called amino acids. Twenty different amino acids are found in most proteins. These amino acids are linked by peptide bonds, forming either short or long chains called, respectively, oligo­peptides or polypeptides. Some proteins, including Ig molecules, con­sist of several polypeptide chains held together by other types of chemical bond. The polypeptide chains in a molecule usually are twisted around each other, giving the molecule the appearance of being round or ovoid. The final shape is one of the factors that deter­mines a protein’s function. This shape, in fact, is determined by the particular linear sequence of amino acids in the polypeptide chains that make up the protein molecule. The information that determines the amino-acid sequence of every protein the body makes is encoded in the DNA (deoxyribonucleic acid) of the genes of each cell in the body. Translation of this infor­mation begins in the nucleus of a cell. The amino acids are joined together one after the other, beginning at what is called the amino terminal end of the molecule and proceeding to the carboxyl terminal end. One of the most exciting avenues of research today is the search for the mechanism that controls the sequence of amino acids in the antibody molecule. The complexity of antibody structure makes it apparent that a complex mechanism is involved. The best-known fact about the structure of antibodies is that, in contrast to most other proteins, antibodies do not consist of a uni­form set of molecules. An antibody actually is a set of molecules that differ in a number of ways but which bind to the antigen that induced their synthesis. The key to understanding the heterogeneity of an antibody was discovered in investigations of the structure of a spe­cial type of immunoglobulin called myeloma protein. Sometimes, in both humans and other animals, tumors of plasma cells, called plas­macytomas, cause a disease called multiple myeloma. Apparently, all the plasmacytoma cells in an individual suffering from multiple myeloma arise from a single plasma cell that somehow was trans­formed into a tumor cell. This tumor cell, along with its progeny, synthesize only one type of Ig molecule. Because the cells secrete this molecule into the serum in great abundance, it is possible for scientists to separate the myeloma immunoglobulin from a person’s serum and subject it to a purification process, which continues until the serum is free of other proteins. The complete sequence of amino acids, as well as a three-dimensional structure of many of these mye­loma proteins, has now been determined. Fortunately, the structure of the myeloma Ig turns out not to be different from the structure of the normal Ig—that is, the normal antibody. From this research five major classes of immunoglobulin have been isolated: IgA, IgD, IgE, IgG, and IgM. Immunoglobulin G, pres­ent in the highest concentration in serum, has been studied the most extensively. Its structure serves as a model of the other classes of immunoglobulin. The IgG molecule shown in Figure 5.3 contains four polypeptide chains, two identical, heavy (or H) chains, and two iden­tical, light (or L) chains. Each half of the molecule contains an.H and an L chain. The chains are held together by an amino-acid bond (cysteine) on each chain. The weight of the molecule is about 160,000 daltons, one dalton being a unit of mass equal to one-sixteenth of an oxygen atom. The immunoglobulins IgD and IgE have the same four-chain structure. Serum IgA, however, may occur in the same monomeric form, or it may consist of two more units (dimer or polymer) held together by additional interchain bonds. In cer­tain secretions—for example, tears and saliva—IgA occurs in high concentrations. Here, it consists of two basic units joined by a small polypeptide chain called a secretory piece. IgM consists of five of the basic four-chain units joined by another small polypeptide known as a J chain. Its molecular weight is about 900,000 daltons, hence the name macroglobulin. As shown in Figure 5.3, each H or L chain is looped on itself in regions called domains, with four or five in an H chain and two in an L chain. In the IgG molecule, the first domain of each L chain lies opposite the first domain of its adjacent H chain; the same is true of the second domain of each chain. The third domain of an H chain is opposite the third domain of the other chain; this is also true of the fourth domain of each chain. Thus each IgG molecule has six pairs of overlapping domains, small globules within the larger IgG molecule. The first domain of a chain occurs at its amino terminal end, that is, the end synthesized first. Analysis of the amino-acid sequence of a large number of Ig molecules reveals much more sequence varia­bility in the first 110 amino-acid positions, beginning at the amino terminal end, of both the H and the L chains, than in the rest of the molecule. For this reason, this part of the polypeptide chains is called the variable (V) region. The rest of the molecule is the constant (C) region because there is much less variability when many Ig molecules are compared. The V and С regions of an L chain are of approxi­mately equal length, whereas the V region comprises only 20 to 25 percent of the length of the H chain. The heterogeneous nature of the sequence in each V region is most evident in three or four subre gions, each of which contains ten to twelve amino acids. These regions are called hypervariable (hv) regions. Because of the folding of the polypeptide chains in the V regions of the amino terminal H and L domains, the hv regions of each chain are extremely close to each other. In fact, they are situated in a cleftlike pocket near the amino terminal end of each half of the molecule. It is in this cleft that the antigenic determinant of an antigen molecule binds to an antibody molecule; hence, it is called the antigen-binding site of the Ig molecule. There are two identical antigen-binding sites on each IgG molecule; an IgM molecule has ten such sites. It is the particular amino acid sequence in the hv regions of a particular Ig molecule that is responsible for the unique configuration of its combining site; this configuration imparts speci­ficity to the Ig molecule and determines its affinity for its homologous antigen. It has been estimated that a human can synthesize more than a million different combining sites, or unique antibody molecules. Additional heterogeneity becomes apparent when the amino acid sequences of the constant regions are considered. These regions (about one hundred residues long in the L chain and three hundred to four hundred residues long in the H chain) are only relatively constant when different molecules are compared. Nevertheless, except for small differences due to genetic polymorphisms, it is possible to place the C-region sequences in a small number of major groups, two for the L chain (kappa and lambda) and five for the H chain (alpha, delta, epsilon, gamma, and mu). These differences in H-chain sequence are what distinguish the five classes of immunoglobulin. The differences in sequence are also responsible for differences in the function of the major classes of immunoglobulin. For example, IgE is the immunoglobulin respon­sible for allergic reactions. Within these classes, smaller variations in C-region sequences define Ig subclasses. For instance, there are four subclasses of IgG in humans:
IgGl, IgG2, IgG3, and IgG4. To a certain extent, these subclasses differ in their properties. Because no subclasses of IgE have been identified thus far, researchers believe that all IgE molecules have an equal effect in mediating an allergic reaction. Also of interest are the patterns of Ig classes that occur in various immune responses. Usually, IgM antibodies predominate in a primary immune response. In most secondary responses, IgG anti­bodies predominate, but smaller amounts of IgA and IgM antibodies are also produced. Some infectious diseases cause antibodies —mainly IgM—to be produced, whereas other diseases induce the production primarily of IgG antibodies. IgA antibodies predominate in most ex­ternal secretions. IgD antibodies are located on the surface of В lymphocytes; very little ever appears in serum. The production of IgE antibodies seems to be favored when the antigen dose is extremely small, when the dose is absorbed via the respiratory or gastrointestinal tract, or when, in experiments with animals, it is injected after adsorp­tion on alum. To recapitulate, antibody is produced by sets of lymphocytes in response to a specific stimulus, or antigen. Antibodies are capable of binding the antigens that induced the formation of the antibodies. An antibody, in fact, is a collection of different protein molecules, all of which can bind the same antigen. All antibodies belong to the group of serum proteins called immunoglobulins. Only when an immunoglobulin is known to bind a particular antigen is it referred to as an antibody against that antigen, and only a small proportion of the total immunoglobulin in serum can be associated with specific antibody activity; the remainder must be regarded as nonspecific, al­though antigens probably exist with which it will react. There are five major classes of immunoglobulin. An antibody against a given antigen usually contains immunoglobulins that belong to more than one of the five classes, and various antibody pools con­tain different proportions of each immunoglobulin. An antibody of each class has a different set of functions. The principal function of most antibodies is to resist infection; antibodies, however, may also be involved in a pathological reaction —for example, in an allergy.

For simplicity, pathological reactions with an immunologic basis are grouped according to four types, I through IV. Type I — immediate hypersensitivity. An immediate hypersensitivity (anaphylactic) reaction is mediated by the IgE antibody on mast cells and basophil leukocytes. An antigen interacting with cell-bound IgE antibodies causes the release of a group of substances known as mediators. These substances are most directly responsible for the major clinical manifestations of immediate hypersensitivity, the symp­toms of which usually begin within minutes after exposure to an antigen. When this type of reaction occurs in humans, it is called an allergic reaction, or allergy. Hay fever, some forms of asthma and hives (urticaria), anaphylactic shock, and some reactions that occur after insect stings or the ingestion of certain drugs or foods are ex­amples of IgE-mediated hypersensitivity reactions. It is well known that clinical manifestations of IgE-mediated reactions may persist for hours, days, or even weeks. When this occurs, additional pathological mechanisms must be invoked. Type II — cytotoxic reactions. Cytotoxic reactions are those in which an IgG antibody directed against a cell membrane component reacts with the component and, in so doing, activates the body’s com­plement system. The complement system consists of interacting pro­teins (some of which are enzymes) in series which, when activated, react in a cascade manner. The end product of this reaction is the destruction of the membrane of the cells or. the bacteria. The unto­ward reaction following a transfusion of incompatible blood is one example of an IgG antibody-mediated cytotoxic reaction. Type III — immune complexes. Type III reactions are those in which immune complexes form. That is, aggregates containing many molecules of antigen and antibody, mainly of the IgG and IgM classes, are formed. Immune complexes can become localized in small blood vessels, where they activate the complement system, producing local, acute inflammation. When an antigen is injected into the skin of a sensitized animal, an area of local swelling and erythema appears The complement system consists of a series of interacting enzymes which can be activated in either of two ways. In one, known as the classical pathway, the system is turned on when antibody reacts with an antigen such as a red blood cell (RBC), forming an immune complex. This com­plex binds CI, the first enzyme in the series, and the cascade reaction begins. In the alternate pathway, certain substances — such as the toxins (poisons) produced by bacteria — trigger the latter part of the cascade.
after one to two hours. This condition will peak in three to six hours; after that, it usually subsides. This is known as the Arthus reaction. Serum sickness is a more generalized immunopathology (vasculitis) caused by circulating immune complexes. Some reactions to drugs are mediated by the immune-complex mechanism. Type IV — delayed hypersensitivity. The tuberculin test is the clas­sic example of a type IV reaction. In this type, a local reaction occurs in the skin twenty-four to forty-eight hours after injection of tuber­culin in a person sensitive to tuberculin. This reaction is also known as a cell-mediated immune response. Sensitized T cells interact with antigen, proliferate, then secrete substances called lymphokines. Lym-phokines draw other cells, principally monocytes, from the blood to the site of the antigen. The local reaction, then, results from the in teraction of monocytes (which are transformed into macrophages] with the localized deposit of antigen. This reaction can produce im­munity in the case of an infectious agent, such as Mycobacterium, the cause of tuberculosis. Rejection of tissue or an organ transplant from one individual to another is also an example of cell-mediated re­action.

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.

Lymphocytes are derived from precursor cells called stem cells that originate in the yolk sac, liver, and bone marrow of the fetus. They circulate in the blood and lymph fluid, finally lodging in the main lymphatic organs. Bone marrow and the thymus are the primary lymphoid organs; the spleen, lymph nodes, and lymphoid masses asso­ciated with the gastrointestinal and respiratory membranes constitute the secondary, or peripheral, lymphoid organs. Secondary lymphoid organs contain an intricate network of col­lagen fibers, to which phagocytic cells are attached and in which lym­phocytes mix thoroughly as they move along. This process permits the lymphoid organs to act as efficient traps for foreign material that has gained access to the body. These organs are also well adapted for interaction between lymphocytes and the foreign matter. The stem cells that remain in the bone marrow after birth are pluripotent; they continue to give rise not only to more stem cells but to all types of red and white blood cells, including lymphocytes. Moreover, they retain this ability as a person matures and becomes an adult. Those stem cells destined to become lymphocytes mature in one of two ways: they become either "T" cells or "B" cells. During the pre- and postnatal parts of their lives, the precursors of T cells migrate from the bone marrow to the thymus (a small organ at the base of the neck), where they establish themselves and become mature T cells. For reasons that are not yet clear, as many as 90 percent of the thymic lymphocytes die within the thymus. It has been hypothesized that these are cells destined to react with "self-antigens," which, by this means, were eliminated in an attempt to avoid an autoimmune reaction. Whatever the reason, some mature T cells eventually leave the thymus and enter the body’s circulatory system. In the blood, they constitute about 75 percent of the circu­lating lymphocytes, or about one-third of all the white blood cells in circulation. The cells can be recognized and counted by virtue of spe­cial "markers" on their surface. For example, human T cells can bind the red blood cells of a sheep to their surface. When human white blood cells are mixed with a sheep’s red blood cells, the sheep’s cells form small clusters around the T cells, which allows ready identifi­cation of the T cells. In a process called cell-mediated immunity, the T cells play a major role in the body’s resistance to viruses, fungi, and intracellular bacteria (for example, those that cause tuberculosis), to protozoan parasites such as those that cause African sleeping sickness, in the rejection of foreign tissue or organ transplants, and in immunity to tumors. When mature T cells are stimulated by an appropriate stimu­lus as an antigen, they proliferate and secrete chemical substances called lymphokines. The most important lymphokines are the macro­phage migration-inhibition factor (MIF), the macrophage-activating factor (MAF), interferon, the T cell-replacing factor, and several fac­tors that attract white blood cells. A macrophage is a scavenger (phagocytic) cell derived from a monocyte, a particular kind of white blood cell. Macrophages are found throughout the body, but they are more likely to be found where there is chronic inflammation. They vigorously ingest and de­stroy numerous foreign substances and microorganisms. In contrast to T cells, which appear to respond to a limited number of antigens, macrophages ingest antigens more or less indiscriminately. In other words, it is the T cells, and not macrophages, that recognize a sub­stance or microorganism as foreign. When a foreign substance invades the body, a group (or subset) of T cells identifies the invader and produces lymphokines, which then bring the macrophages to the site of the invasion. There the macrophages attempt to halt and destroy the foreign substance. The body’s response is the same whether the substance is harmful (in the case of tuberculosis bacteria), helpful (a kidney transplant), or indifferent (poison ivy). There are several different types of T cells in addition to those involved in cell-mediated immunity. One type, known as a cytotoxic T cell, attacks and destroys almost all foreign cells, including tumor cells, which, though not exactly foreign, differ sufficiently from nor­mal cells for the immune system to recognize them as foreign. Two other types of T cell, "helper" T cells and "suppressor" T cells, help regulate the production of antibodies. Lymphocytes, particularly T lymphocytes, can circulate throughout the blood and lymph and peripheral tissues, and then recirculate. They also have a lengthy life span, living as long as twenty years. Because of their circulation ability and longevity, they are superbly equipped to play a major part in the recognition and response functions of adaptive immunity. В cells begin to mature in the bone marrow. At some point, they leave the marrow and migrate to the peripheral lymphatic tissues and organs (Figure 5.1), where they are in a good position to interact with antigens, T cells, and macrophages.

The body’s immunologic mechanisms are central to what are com­monly regarded as allergic diseases. In orderlo properly diagnose and manage these diseases and the immunologic reactions to them, we must first have some idea of what immunology is. Immunology deals with the organs, cells, and molecules responsible for recognizing and disposing of foreign (or "nonself") substances that enter the body, with the response to these substances, with interactions between the products of the response and the substances, and with the means of manipulating a response to therapeutic advantage. The science of immunology arose from a need to study the body’s resistance to infection. This resistance is made possible by two types of mechanism, natural immunity and adaptive immunity. In natural immunity, the inborn mechanisms present in all healthy members of a species protect the body, whereas in adaptive immunity, protection is provided by alterations in the immune system induced by encoun­ters between the immune system and specific infectious organisms. Both natural and adaptive mechanisms also come into play when the foreign substance is not infectious. Adaptive immune mechanisms operate through such components of natural immunity as phagocytic cells (found in both the blood and the tissues), eosinophilic and basophilic leukocytes (two types of blood cell), mast cells, clotting factors, and the complement system —all of which are particularly important when an allergic reaction is involved. The principal cells of the immune system are the lymphocytes. Lymphocytes respond to foreign substances called antigens. The lymphocytes can alter their response when the same antigen is en­countered again; thus they are adaptive. In subsequent encounters with an infectious antigen —for example, with measles virus—no clini­cally obvious reaction may be detectable. That is, a state of immunity to that particular virus has been established. On the other hand, sub­sequent encounters may result in an inflammatory reaction that is more intense than the original one. Poison ivy is one such response. When a reaction is harmful or is exaggerated, as in the case of poison ivy, a person is said to have a hypersensitivity reaction. Allergies are a type of hypersensitivity reaction mediated by the immune system and involving components of natural immunity. Let us now look at these components and their interaction in some detail.

The measurement of serum IgE levels, first made possible by the discovery in 1967 of a patient with IgE myeloma, is of interest to physicians because elevations in serum IgE levels occur more fre­quently in allergic individuals. The reader should remember, though, that the total serum IgE measurement is not a specific diagnostic test for allergy. Subsequent purification of IgE, and the preparation of antibody specific for human IgE (anti-IgE), led to the development of labora­tory procedures for measuring total IgE and allergen-specific IgE. These new diagnostic methods supplement the traditional skin test, which gives only partially complete quantitative information about specific IgE antibody. Several methods, called radioimmunoassay methods, are now available for measuring total serum IgE. The competitive radioimmunosorbent test (RIST) is a competitive-binding assay in which IgE in the test sample competes with IgE that has been tagged with radio­active iodine. In this test, the IgE content of the serum is inversely proportional to the radioactivity bound by the anti-IgE. The absolute level of IgE is obtained by comparing it with a standard curve derived from a serum containing a known amount of IgE. The RIST method has limited sensitivity, however, primarily because of the variability among samples containing less than 100 nanograms of IgE per milliliter. In the noncompetitive RIST method, a sample of test serum is incubated with solid-phase anti-IgE in the absence of labeled IgE. The solid-phase anti-IgE is then washed and incubated with anti-IgE labeled with radioactive iodine, which, in turn, binds to the IgE on the solid phase. The radioactivity binds in proportion to the amount of IgE bound during the first incubation, and is directly proportional to the IgE content of the serum sample. Because of improvements in sensitivity and precision, this procedure is now the method of choice for measuring serum IgE. Of perhaps greater interest is the measurement of specific IgE antibody using the radioallergosorbent test (RAST). This test is similar to the noncompetitive RIST, except that allergen, rather than anti-IgE, is bound to the solid-phase matrix. After incubation of solid-phase allergen (allergosorbent) with serum, the matrix is washed and then incubated with labeled anti-IgE. The amount of radioactivity that binds to the allergosorbent is directly proportional to the allergen-specific IgE antibody in the serum sample. The result is expressed as "antibody titer" relative to that of a designated reference serum. Re­cent modifications of the RAST method permit quantification of abso­lute amounts of IgE antibody specific for several purified allergens. But interpretation of the results is still uncertain, especially when the values obtained are not highly elevated because of a fairly wide over­lap of nonallergic and allergic individuals in the lower range of values. RAST, therefore, is still mainly a semiquantitative, diagnostic screen­ing procedure. The more sensitive skin test still has greater diagnostic significance, particularly when the test results are borderline. Two types of cell must be considered in discussing allergic reac­tions: mast cells and basophil leukocytes. Mast cells, located near the small blood vessels, are found in connective tissue throughout the body. This is clearly a convenient location for cells associated with the functioning of the blood vessels. We do not yet know exactly where mast cells originate, although evidence recently uncovered sug­gests that some mast cells come from the bone marrow, while others are derived from lymphocytes. If we are to understand the role of mast cells in allergic reactions, we must learn more about their origin. One thing we do know is that mast cells are long-lived, though just how long is uncertain. Basophil leukocytes are the progeny of the same stem cells (located in the bone marrow) that produce such other types of white blood cell as neutrophils and eosinophils. As basophils mature (perhaps under the influence of T cells), they leave the bone marrow and enter the blood, where they appear to remain for only one or two days. Then the basophils leave the blood and enter the extravascular spaces, most often at sites of allergic reactions or where parasites have lodged. Many apparently exit the body by moving into the respiratory or the gastrointestinal tract. Both mast cells and basophils have an abundance of dense granules in their cytoplasm. These cells can be identified in the light microscope only when the granules are specially stained. Each granule is sur­rounded by a membrane that effectively insulates the contents of the granule from the rest of the cell. Granules contain a high concentra­tion of histamine, which is held there because it is bound to the anti­coagulant heparin and to a protein matrix. IgE molecules have a rather high affinity for the surface of mast cells and basophils. This affinity is due to the presence of molecules (receptors) on the cell’s surface, which recognize certain unique structural features of the constant region of the IgE molecule. In both normal and allergic people, IgE molecules are bound to mast cells and basophils. In allergic individuals, however, some of these molecules are antibody molecules specific to one or more allergens. Thus the cells in an allergic person are said to be sensitized; when allergen reaches them, they respond characteristically. An allergic reaction is said to begin when an allergen molecule interacts with two specific IgE antibody molecules on the surface of a sensitized mast cell or a basophil. Exactly how the bridging of two antibody molecules by an allergen induces the cell to respond is still not known. What is known is that the cell can be triggered in the absence of either antigen or antibody. All that must be done is to bridge two of the surface receptor molecules; this is true even when IgE is not present. The triggering process has recently been accom­plished with antibody specific for the mast cell receptor itself. A number of biochemical changes have been detected in mast cells and basophils immediately after they are triggered by the interaction of allergen with IgE antibody. The change most thoroughly studied oc­curs in the cyclic nucleotide system, in which the cyclic adenosine monophosphate (cAMP) concentration falls, while that of the cyclic guanosine monophosphate (cGMP) rises. A fall in the cAMP-cGMP
ratio favors the entry of calcium ions into the cell. In a process called degranulation, an increase in the concentration of intracellular cal­cium leads immediately to secretion of the contents of the granule from the cell into the surrounding body fluid. This process, however, does not result in destruction of the cell. Degranulation operates thus: triggering of the cell causes the cell’s surface membrane to move up against the membrane surrounding the granules (which are just be­neath the surface). The two membranes quickly fuse, and an opening forms at the site. As the opening enlarges, the interior of the granule is exposed to the fluid outside the cell. The contents of the granule are now effectively outside the cell. Note that the membrane of the cell remains intact, with the membrane of the granule now incorporated in it. At this point, the histamine quickly dissolves out of the granule and diffuses into the surrounding tissue. There it acts directly on ad­jacent small blood vessels, making them leaky to the fluid of the blood. This, in turn, leads to edema, or swelling of the tissues, and acts on the nerve endings, causing an itching sensation. When this happens in the skin, it is called hives. The process is the same that occurs when a doctor performs a skin test on an allergic patient, using a dilute solution of allergenic extract. A local reaction appears within a few minutes, then fades within
twenty to thirty minutes. This is an immediate hypersensitivity reaction. Histamine also acts on smooth, or nonskeletal, muscle —for exam­ple, on the tiny muscles in the bronchial tree—causing them to con­tract and making the air passages in the lungs narrower. This condition is called bronchospasm, or asthma. Histamine can also act on mucous glands, causing the secretion of thin, watery mucus—a condition that usually occurs in the noses of allergic people during the hay fever season. Histamine is only one of the "mediators" of anaphylaxis released when mast cells or basophils are triggered by the interaction of aller­gens and IgE. Other substances that are released, for which there is evidence of a role in allergic inflammation, are: 1. SRS-A, a slow-reaction substance of anaphylaxis, known as leukotriene, that was recently identified as a product of arachidonic acid metabolism. It acts on smooth muscles. 2. ECF-A, an eosinophil chemotactic factor of anaphylaxis that consists of several different short peptide chains. It acts as an attrac-tant to eosinophils. 3. PAF, a factor recently identified as an acetyl glycerylether compound. It activates platelets, small blood corpuscles that con­tain additional mediators, and anticoagulants.
Other mediators (see Figure 5.7) may be involved in allergic re­actions, but their role is unclear. The explosive release of mediators from mast cells and basophils depends on where the reaction occurs. Most allergic reactions occur in the eyes, nose and throat, lungs, intestines, or skin. In a condition known as anaphylactic shock, the entire body is affected. The intensity of an allergic reaction depends somewhat on the concentration of mediators that reach their targets in the tissues and on how long-acting they are. Powerful inactivators of each mediator occur in the blood and the tissues, limiting this concentration and duration. To a certain degree, the intensity and duration of a particular allergic reaction depend on the number of mast cells and basophils in the tissues at the time of the allergen-IgE antibody interaction. Their number is not constant in the target tissues. During the ragweed-pollen season, for example, mast cells and basophils appear in greater numbers in the membrane that lines the nasal cavity, as well as in the abundant mucus typical of hay fever. When these cells are con­centrated in the nose, much more mediator exists to be released after pollen is inhaled. This condition may have something to do with the fact that symptoms of ragweed hay fever can continue for days or weeks after the actual ragweed pollen season has ended. We do not know exactly what causes the influx of these cells during the pollen season, or what accounts for their disappearance after the season. It will not be surprising, though, if it turns out that T cells have some­thing to do with the influx of mast cells and basophils.

Antigens are substances that induce a specific immune response. The clearest" response is the appearance of antibodies in blood serum and other body fluids. The antibodies produced in this response rec­ognize the antigen that stimulated their production; thus they can neutralize the antigen’s harmful effects, if any, hastening elimination of the antigen from the body. How is this complex sequence of events set in motion? When an antigen enters an animal, some of it is absorbed by the ever-present mac rophages. The macrophages "process" the antigen, presumably enhancing the ability of some of the antigen to interact with lympho­cytes. The macrophages then present the antigen to T and В cells; В cells bear a surface receptor that recognizes a specific region of the antigen, called the determinant. The В cells react with the antigen, thereby initiating an immune response. The B-cell receptor is, in fact, an antibody, the same one the В cell will eventually manufacture. Each В cell has only one type of antibody on its surface; each В cell (and its progeny, or "daughter," cells) can produce only that anti­body molecule. Protein antigens may have several different deter­minants; this enables them to stimulate many different В cells to di­vide, to differentiate, and to generate clones (identical daughter cells) of antibody-secreting plasma cells. Again, each cell secretes an antibody identical to the receptor on the В cell from which it was derived. Moreover, each unique antigenic determinant on an antigen can elicit antibodies that vary in their affinity (or ability to bind) and specificity (ability to recognize a particular antigen and distinguish it from the others). The quantity and quality of antibody produced in response to a given antigen depend largely on the nature and amount of the antigen, the route it takes in the body, and the form in which it appears. In the case of pollen allergy, for example, natural exposure to the minute amounts of antigen present in inhaled pollen grains causes a signifi­cant, though still minute, amount of an antibody to be produced. This antibody is called reaginic, or IgE, antibody. In contrast, when an extract of pollen containing relatively large amounts of the same antigen is injected for treatment, relatively large amounts of an anti­body that blocks the allergic reaction will be produced. We are just beginning to understand the difference between regulation of the В cells that make the reaginic antibody and regulation of the В cells that make the blocking, or IgG, antibody. For one thing, helper T cells and suppressor T cells act differently on the two groups of В cells. It is well known that genetic factors influence immune responses; for example, allergies occur more often in some families than in others. Moreover, some genes—called immune response genes—are dedicated to controlling the response to particular antigens, whereas some control the type and amount of an antibody that can be made, some the number and distribution of lymphocytes, and some the cell-surface molecules (such as transplantation antigens) involved in the recognition of antigens. In an animal that has never encountered a particular antigen, there resides a small population of В cells committed to respond to that antigen by virtue of its specific surface receptor. When the antigen enters the body and encounters the appropriate cells, it causes these cells to divide. Some differentiate and become antibody-secreting plasma cells. The concentration of antibody in the serum rises abruptly, and the presence of this antibody increases the rate of anti­gen elimination. But plasma cells survive only a few weeks. As they die, the serum antibody level subsides, ending this primary immune response. Some В cells, however, survive as an expanded set of specifically committed, long-lived, "memory" В cells. The animal mentioned at the beginning of this paragraph is now primed for a heightened, more rapid, secondary response to the same antigen. Here we come to the explanation for the two principal characteristics of the immune response—memory and specificity. The initial encounter with an antigen leaves the animal (or human) with a greatly ex- panded population of lymphocytes (both T cells and В cells) that can respond when that antigen is encountered again.