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4466_Ch01_001-015 30/08/16 5:01 PM Page 1 Nature of the I Immune System 1 4466_Ch01_001-015 30/08/16 5:01 PM Page 2...

4466_Ch01_001-015 30/08/16 5:01 PM Page 1 Nature of the I Immune System 1 4466_Ch01_001-015 30/08/16 5:01 PM Page 2 1 Introduction to Immunity and the Immune System Christine Dorresteyn Stevens, EdD, MT(ASCP) LEARNING OUTCOMES CHAPTER OUTLINE After finishing this chapter, you should be able to: IMMUNITY AND IMMUNIZATION 1. Discuss how immunology as a science began with the study INNATE VERSUS ADAPTIVE IMMUNITY of immunity. CELLS OF THE INNATE IMMUNE 2. Describe what is meant by an attenuated vaccine. SYSTEM 3. Explain how the controversy over humoral versus cellular immunity Leukocytes in Peripheral Blood contributed to expanding knowledge in the field of immunology. Tissue Cells 4. Distinguish innate from adaptive immunity. CELLS OF THE ADAPTIVE IMMUNE 5. Describe the types of white blood cells (WBCs) capable of SYSTEM phagocytosis. B Cells 6. Explain the role of tissue cells in immunity. T Cells 7. Discuss how natural killer (NK) cells differ from T lymphocytes. Natural Killer (NK) Cells 8. Identify the two primary lymphoid organs and discuss the main ORGANS OF THE IMMUNE SYSTEM functions of each. Primary Lymphoid Organs 9. List four secondary lymphoid organs and discuss their overall Secondary Lymphoid Organs importance to immunity. SUMMARY 10. Describe the function and architecture of a lymph node. CASE STUDIES 11. Compare a primary and a secondary follicle. REVIEW QUESTIONS 12. Explain the makeup of a cluster of differentiation. 13. Differentiate the roles of T cells and B cells in the immune response. You can go to DavisPlus at davisplus.fadavis.com keyword Stevens for the laboratory exercises that accompany this text. 2 4466_Ch01_001-015 30/08/16 5:01 PM Page 3 Chapter 1 Introduction to Immunity and the Immune System 3 KEY TERMS Adaptive immunity Dendritic cells Lymphocyte Plasma cells Antibodies Diapedesis Macrophages Primary follicles Antigens Eosinophils Mast cells Primary lymphoid organs Attenuation Germinal center Memory cells Secondary follicles Basophils Humoral immunity Monocytes Secondary lymphoid organs Bone marrow Immunity Natural killer (NK) cells Spleen Cell-mediated immunity Immunology Neutrophil Thymocytes Chemotaxins Innate (natural) immunity Periarteriolar lymphoid Thymus Clusters of differentiation (CD) Leukocytes sheath (PALS) Cytokines Lymph nodes Phagocytosis Although humans have been trying for many centuries to un- The next major development in disease prevention did not ravel the secrets of preventing disease, the field of immunol- occur until almost a hundred years later when Louis Pasteur, ogy is a relatively new science. Immunology can be defined often called the father of immunology, observed by chance that as the study of a host’s reactions when foreign substances are older bacterial cultures would not cause disease in chickens introduced into the body. Such foreign substances that induce (Fig. 1–1).2,3 Subsequent injections of more virulent organisms a host response are called antigens. Antigens are all around had no effect on the birds that had been previously exposed to us in nature and they vary from substances such as pollen that the older cultures. In this manner, the first attenuated vaccine may make us sneeze to serious bacterial pathogens such as was discovered; this event can be considered the birth of im- Staphylococcus aureus or Group A Streptococcus that can cause munology.3 Attenuation, or change, means to make a pathogen life-threatening illnesses. The study of immunology has given less virulent; it takes place through heat, aging, or chemical us the ability to prevent diseases such as smallpox, polio, means. Attenuation remains the basis for many of the immuniza- diphtheria, and measles through the development of vaccines. tions that are used today. Pasteur applied this same principle of In addition, understanding how the immune system works attenuation to the prevention of rabies in affected individuals. has made successful organ transplantation possible and has given us new tools to treat diseases such as cancer and certain autoimmune diseases. Immunological techniques have affected testing in many areas of the clinical laboratory and allowed for such testing to be more precise and automated. Thus, the study of immunology is important to many areas of medicine. In this chapter, we will provide a brief look at the history of the field and then introduce the cells and tissues of the immune system to form a basis for understanding how the immune sys- tem works. In later chapters we will apply this knowledge to principles of testing for specific diseases. Immunity and Immunization Immunology as a science has its roots in the study of immunity: the condition of being resistant to infection. The first recorded attempts to deliberately induce immunity date back to the 1500s when the Chinese inhaled powder made from smallpox scabs in order to produce protection against this dreaded dis- ease. The hypothesis was that if a healthy individual was ex- posed as a child or young adult the effects of the disease would be minimized. However, the early exposure did not always work. Further refinements did not occur until the late 1700s when an English country doctor by the name of Edward Jenner was able to successfully prevent infection with smallpox by injecting a more harmless substance—cowpox—from a disease affecting cows.1 Details of the development of this first vaccine FIGURE 1–1 Louis Pasteur. (Courtesy of the National Library can be found in Chapter 25. of Medicine.) 4466_Ch01_001-015 30/08/16 5:01 PM Page 4 4 SECTION 1 Nature of the Immune System Innate Versus Adaptive Immunity There are five principal types of leukocytes in peripheral blood: neutrophils, eosinophils, basophils, monocytes, and In the late 1800s, scientists turned to identifying the actual lymphocytes. The first four types are all part of innate im- mechanisms that produce immunity in a host.2 Elie Metchnikoff, munity. Because lymphocytes are considered part of adaptive a Russian scientist, observed under a microscope that foreign immunity, they will be considered in a separate section. objects introduced into transparent starfish larvae became sur- Several cell lines that are found in the tissues, namely mast rounded by motile amoeboid-like cells that attempted to destroy cells, macrophages, and dendritic cells, will also be discussed the penetrating objects. This process was later termed phagocy- in this chapter because they all contribute to the process of tosis, meaning cells that eat cells.2,4 He hypothesized that im- immunity. munity to disease was based on the action of these scavenger All blood cells arise from a type of cell called a hematopoi- cells and was a natural, or innate, host defense.4 etic stem cell (HSC). To form WBCs, the HSC gives rise to Other researchers contended that noncellular elements in two distinct types of precursor cells: common myeloid pre- the blood were responsible for protection from microorgan- cursors (CMP) and common lymphoid precursors (CLP). isms. Emil von Behring demonstrated that diphtheria and CMPs give rise to the WBCs that participate in phagocytosis, tetanus toxins, which are produced by specific microorganisms which are known as the myeloid line. Phagocytic cells are as they grow, could be neutralized by the noncellular portion key to innate immunity, but they are also important in pro- of the blood of animals previously exposed to the microorgan- cessing antigens for the adaptive response. Lymphocytes isms. The theory of humoral immunity was thus born and arise from CLPs and form the basis of the adaptive immune sparked a long-lasting dispute over the relative importance of response. Mature lymphocytes are found in the tissues cellular versus humoral immunity. as well as in peripheral blood. Refer to Figure 1–2 for a In 1903, an English physician named Almroth Wright simplified scheme of blood cell development, known as linked the two theories by showing that the immune response hematopoiesis. involved both cellular and humoral elements. He observed that Neutrophils certain humoral, or circulating, factors called opsonins acted to The neutrophil, or polymorphonuclear neutrophilic (PMN) coat bacteria so that they became more susceptible to ingestion leukocyte, represents approximately 50% to 75% of the total by phagocytic cells.2 These serum factors include specific pro- peripheral WBCs in adults.5 These are around 10 to 15 μm in teins known as antibodies, as well as other factors called acute- diameter with a nucleus that has between two and five lobes phase reactants that increase nonspecifically in any infection. (Fig. 1–3). Hence, they are often called segmented neu- Antibodies are serum proteins produced by certain lympho- trophils, or “segs.” They contain a large number of neutral cytes when exposed to a foreign substance and they react staining granules when stained with Wright stain, two-thirds specifically with that foreign substance (see Chapter 5). of which are specific granules; the remaining one-third are These discoveries showed that there were two major called azurophilic granules.6 Azurophilic or primary granules branches of immunity, currently referred to as innate immunity contain antimicrobial products such as myeloperoxidase, and adaptive immunity. Innate, or natural immunity, is the in- lysozyme, elastase, proteinase-3, cathepsin G, and defensins, dividual’s ability to resist infection by means of normally present which are small proteins that have antibacterial activity.5 Spe- body functions. These are considered nonadaptive or nonspe- cific granules, also known as secondary granules, contain cific and are the same for all pathogens or foreign substances to lysozyme, lactoferrin, collagenase, gelatinase, and respiratory which one is exposed. No prior exposure is required and the burst components.5,7 See Chapter 3 for a discussion of the response lacks memory and specificity. Many of these mecha- oxidative burst, which takes place during phagocytosis. The nisms are subject to influence by such factors as nutrition, age, main function of neutrophils is phagocytosis, resulting in fatigue, stress, and genetic determinants. Adaptive immunity, in the destruction of foreign particles.6 contrast, is a type of resistance that is characterized by speci- Normally, half of the total neutrophil population in periph- ficity for each individual pathogen, or microbial agent, and the eral blood is found in a marginating pool adhering to blood ability to remember a prior exposure. Memory and specificity vessel walls, whereas the rest circulate freely for approximately result in an increased response to that pathogen upon repeated 6 to 8 hours.5 There is a continuous interchange, however, be- exposure, something that does not occur in innate immunity. tween the marginating and the circulating pools. Margination Both systems are necessary to maintain good health. In fact, occurs to allow neutrophils to move from the circulating blood they operate in combination and are dependent upon one an- to the tissues through a process known as diapedesis, or other for maximal effectiveness. Certain key cells are considered movement through blood vessel walls. They are attracted to a essential to both systems and they will be discussed next. specific area by chemotactic factors. Chemotaxins are chemi- cal messengers that cause cells to migrate in a particular direc- tion. Once in the tissues, neutrophils have a life span of up to Cells of the Innate Immune System several days. Normally, the influx of neutrophils from the bone marrow equals the output from the blood to the tissues to Leukocytes in Peripheral Blood maintain a steady state. However, in the case of acute infection White blood cells (WBCs), or leukocytes, in the peripheral an increase of neutrophils in the circulating blood can occur blood play a key role in both innate and adaptive immunity. almost immediately.8 4466_Ch01_001-015 30/08/16 5:01 PM Page 5 Chapter 1 Introduction to Immunity and the Immune System 5 T cells NK cell B cell FIGURE 1–3 Neutrophils. (From Harmening D. Clinical Hematology and Fundamentals of Hemostasis. 5th ed. Philadelphia, PA: F.A. Davis; 2009. Fig. 1–4.) CLP Dendritic cell Monocyte HSC Neutrophil Eosinophil FIGURE 1–4 Eosinophil. (From Harmening D. Clinical Hematology and Fundamentals of Hemostasis. 5th ed. Philadelphia, PA: F.A. Davis; 2009. Fig. 1–6.) CMP Basophil reddish-orange granules. Granules in eosinophils, which are spherical and evenly distributed throughout the cell, contain a large number of previously synthesized proteins including Erythrocytes catalase, lysozyme, cytokines (chemical messengers), growth factors, and cationic proteins.5,9 Eosinophils are capable of phagocytosis but are much less Platelets efficient than neutrophils because they are present in smaller numbers and they lack digestive enzymes. Eosinophils are able FIGURE 1–2 Simplified scheme of hematopoiesis. In the marrow, to neutralize basophil and mast cell products. In addition, they hematopoietic stem cells (HSC) give rise to two different lines—a can use cationic proteins to damage cell membranes and kill common lymphoid precursor (CLP) and a common myeloid precur- larger parasites that cannot be phagocytized. (See Chapter 22 sor (CMP). CLPs give rise to T/NK progenitors, which differentiate for details.) However, the most important role of eosinophils into T and NK cells, and to B-cell progenitors, which become is regulation of the immune response, including regulation of B cells and dendritic cells. The CMP differentiates into neutrophils, mast cell function.5 monocytes/macrophages, eosinophils, basophils, erythrocytes, and platelets. Basophils Basophils are the least numerous of WBCs found in periph- eral blood, representing less than 1% of all circulating WBCs. Eosinophils The smallest of the granulocytes, basophils are slightly larger Eosinophils are approximately 12 to 15 μm in diameter and than RBCs (between 10 to 15 μm in diameter) and contain normally make up between 1% and 3% of the circulating coarse, densely staining deep-bluish-purple granules that WBCs in a nonallergic person. Their number increases in an often obscure the nucleus5,9 (Fig. 1–5). Constituents of allergic reaction or in response to certain parasitic infections. these granules include histamine, cytokines, growth factors, The nucleus is usually bilobed or ellipsoidal and is often ec- and a small amount of heparin, all of which have an impor- centrically located (Fig. 1–4). Eosinophils take up the acid tant function in inducing and maintaining allergic reac- eosin dye and the cytoplasm is filled with large orange to tions.5,8 Histamine contracts smooth muscle and heparin is 4466_Ch01_001-015 30/08/16 5:01 PM Page 6 6 SECTION 1 Nature of the Immune System Tissue Cells Macrophages All macrophages arise from monocytes, which can be thought of as macrophage precursors because additional differentiation and cell division takes place in the tissues. The transition from monocyte to macrophage in the tissues is characterized by progressive cellular enlargement to between 25 and 80 μm.8 Unlike monocytes, macrophages contain no peroxidase.8 Tissue distribution appears to be a random phenomenon. Macrophages have specific names according to their partic- ular tissue location. Macrophages in the lung are alveolar macrophages; in the liver, Kupffer cells; in the brain, microglial FIGURE 1–5 Basophil. (From Harmening D. Clinical Hematology and cells; in the bone, osteoclasts; and in connective tissue, histio- Fundamentals of Hemostasis. 5th ed. Philadelphia, PA: F.A. Davis; 2009. cytes. Macrophages may not be as efficient as neutrophils in Fig. 1–7.) phagocytosis because their motility is slow compared with that of the neutrophils. Some macrophages progress through the tissues by means of amoeboid action, whereas others are im- an anticoagulant. In addition, basophils regulate some mobile. However, their life span appears to be in the range of T helper (Th) cell responses and stimulate B cells to produce months rather than days. the antibody IgE.5,10 Basophils have a short life span of only Macrophages play an important role in initiating and regu- a few hours in the bloodstream; they are then pulled out and lating both innate and adaptive immune responses. Their innate destroyed by macrophages in the spleen. immune functions include microbial killing, anti-tumor activity, intracellular parasite eradication, phagocytosis, and secretion of Monocytes cell mediators. Killing activity is enhanced when macrophages Monocytes are the largest cells in the peripheral blood with a become “activated” by contact with microorganisms or with diameter that can vary from 12 to 22 μm (the average is chemical messengers called cytokines, which are released by 18 μm).9 One distinguishing feature is an irregularly folded or T lymphocytes during the immune response. (See Chapter 6 for horseshoe-shaped nucleus that occupies almost one-half of the a complete discussion of cytokines.) Macrophages play a major entire cell’s volume (Fig. 1–6). The abundant cytoplasm stains role in the adaptive immune response by presenting antigens to a dull grayish blue and has a ground-glass appearance because T and B cells. of the presence of fine dustlike granules. These granules are Mast Cells actually of two types. The first type contains peroxidase, acid phosphatase, and arylsulfatase, indicating that these granules Tissue mast cells resemble basophils, but they come from a dif- are similar to the lysosomes of neutrophils.8 The second type ferent lineage. Mast cells are distributed throughout the body in of granule may contain β-glucuronidase, lysozyme, and lipase, a wide variety of tissues such as skin, connective tissue, and the but no alkaline phosphatase. Digestive vacuoles may also be mucosal epithelial tissue of the respiratory, genitourinary, and observed in the cytoplasm. Monocytes make up between 4% digestive tracts.5 Mast cells are larger than basophils with a small and 10% of total circulating WBCs; however, they do not re- round nucleus and more granules (Fig. 1–7). Unlike basophils, main in the circulation for long. They stay in peripheral blood they have a long life span of between 9 and 18 months.11 The for up to 30 hours; they then migrate to the tissues and become known as macrophages.5 FIGURE 1–6 Two monocytes. (From Harmening D. Clinical FIGURE 1–7 Mast cell. (From Harmening D. Clinical Hematology and Hematology and Fundamentals of Hemostasis. 5th ed. Philadelphia, Fundamentals of Hemostasis. 5th ed. Philadelphia, PA: F.A. Davis; 2009. PA: F.A. Davis; 2009. Fig. 1–13.) Fig. 1–44.) 4466_Ch01_001-015 30/08/16 5:01 PM Page 7 Chapter 1 Introduction to Immunity and the Immune System 7 enzyme content of the granules in mast cells helps to distinguish to 20% of lymphocytes are B cells, 61% to 80% are T cells, and them from basophils because they contain acid phosphatase, 10% to 15% are NK cells.13 alkaline phosphatase, and protease, as well as histamine.5,7,8 The three types of cells are difficult to distinguish visually. Mast cells play a role in allergic reactions, but they can also func- In the laboratory, proteins, or antigens, on cell surfaces can be tion as antigen-presenting cells (APCs). They can both enhance used to identify each lymphocyte subpopulation. In order to and suppress the adaptive immune response. standardize the nomenclature, scientists set up the Human Dendritic Cells Leukocyte Differentiation Antigens Workshops to relate re- search findings.15 Panels of antibodies from different laborato- Dendritic cells are so named because they are covered with ries were used for analysis and antibodies reacting similarly long membranous extensions that make them resemble nerve with standard cell lines were said to define clusters of differ- cell dendrites. They were discovered by Steinman and Cohn entiation (CD). As each antigen, or CD, was found, it was as- in 1973.7 Progenitors in the bone marrow give rise to dendritic signed a number. The list of CD designations currently cell precursors that travel to lymphoid as well as nonlymphoid numbers more than 500.16 Table 1–1 lists some of the most tissue.12 They are classified according to their tissue location important CD numbers used to identify lymphocytes. in a similar manner to macrophages. After capturing an antigen in the tissue by phagocytosis or endocytosis, dendritic cells present the antigen to T lymphocytes to initiate the adaptive B Cells immune response in a similar way as macrophages. Dendritic B cells are derived from a lymphoid precursor that differenti- cells, however, are considered the most effective APC in the ates to become either a T cell, B cell, or NK cell depending on body, as well as the most potent phagocytic cell.13,14 exposure to different cytokines. B cells remain in the environ- ment provided by bone marrow stromal cells. B-cell precursors Cells of the Adaptive go through a developmental process that prepares them for their role in antibody production and, at the same time, re- Immune System stricts the types of antigens to which any one cell can respond. The end result is a B lymphocyte programmed to produce a The key cell involved in the adaptive immune response is the unique antibody molecule. B cells can be recognized by the lymphocyte. Lymphocytes represent between 20% and 40% of presence of membrane-bound antibodies of two types, namely the circulating WBCs. The typical small lymphocyte is similar immunoglobulin M (IgM) and immunoglobulin (IgD). Other in size to RBCs (7–10 μm in diameter) and has a large rounded surface proteins that appear on the B cell include CD19, CD21, nucleus that may be somewhat indented. The nuclear chro- and class II major histocompatibility complex (MHC) mole- matin is dense and tends to stain a deep blue (Fig. 1–8).9 Cy- cules (see Chapter 2).10 toplasm is sparse, containing few organelles and no specific granules, and consists of a narrow ring surrounding the nu- cleus.6 The cytoplasm stains a lighter blue. These cells are T Cells unique because they arise from an HSC and then are further T cells are so named because they differentiate in the thymus. differentiated in the primary lymphoid organs, namely the bone Lymphocyte precursors called thymocytes enter the thymus marrow and the thymus. Lymphocytes can be divided into three from the bone marrow through the bloodstream. As they ma- major populations—T cells, B cells, and natural killer (NK) ture, the T cells express unique surface markers that allow cells—based on specific functions and the proteins on their cell them to recognize foreign antigens bound to cell membrane surfaces. In the peripheral blood of adults, approximately 10% proteins called MHC molecules. The role of T cells is to pro- duce cytokines that contribute to immunity by stimulating B cells to produce antibodies, assisting in killing tumor cells or infected target cells, and helping to regulate both the innate and adaptive immune response. The process is known as cell- mediated immunity. Three main subtypes of T cells can be distinguished accord- ing to their unique functions: helper, cytolytic, and regulatory T cells. The subtypes can be identified by the presence of the CD3 marker on their cell surface, and either CD4, or CD8. T cells bearing the CD4 receptor are mainly either helper or regulatory cells, whereas the CD8-positive (CD8+) population consists of cytotoxic T cells. The ratio of CD4+ to CD8+ cells is approximately 2:1 in peripheral blood. Natural Killer (NK) Cells FIGURE 1–8 Typical lymphocyte found in peripheral blood. (From Harr R. Clinical Laboratory Science Review. 4th ed. Philadelphia, A small percentage of lymphocytes do not express the mark- PA: F.A. Davis; 2013. Color Plate 31.) ers of either T cells or B cells. They are named natural killer 4466_Ch01_001-015 30/08/16 5:01 PM Page 8 8 SECTION 1 Nature of the Immune System Table 1–1 Surface Markers on T, B, and NK Cells ANTIGEN MOL WT (KD) CELL TYPE FUNCTION CD3 20–28 Thymocytes, T cells Found on all T cells; associated with T-cell antigen receptor CD4 55 T helper cells, monocytes, macrophages Identifies T helper cells; also found on most T regulatory cells CD8 60–76 Thymocyte subsets, cytotoxic T cells Identifies cytotoxic T cells CD16 50–80 Macrophages, NK cells, neutrophils Low affinity Fc receptor for antibody; mediates phagocytosis CD19 >120 B cells, follicular dendritic cells Part of B-cell coreceptor; regulates B-cell development and activation CD21 145 B cells, follicular dendritic cells Receptor for complement component C3d; part of B-cell coreceptor with CD19 CD 56 175–220 NK cells, subsets of T cells Not known (NK) cells because they have the ability to kill target cells antigens can occur (Fig. 1–9). Secondary lymphoid organs without prior exposure to them. NK cells do not require the include the spleen, lymph nodes, and various types of thymus for development but appear to mature in the bone mucosal-associated lymphoid tissues (MALT). The primary and marrow itself.17,18 NK cells are generally larger than T cells secondary organs are differentiated according to their function and B cells at approximately 15 μm in diameter and contain in both adaptive and innate immunity. kidney-shaped nuclei with condensed chromatin and promi- nent nucleoli. Described as large granular lymphocytes, NK Primary Lymphoid Organs cells make up 10% to 15% of the circulating lymphoid pool and are found mainly in the liver, spleen, and peripheral Bone Marrow blood.5,10 Bone marrow is considered one of the largest tissues in the There are no surface markers that are unique to NK cells, body and it fills the core of all long flat bones. It is the main but they express a specific combination of antigens that can be source of hematopoietic stem cells, which develop into ery- used for identification. Two such antigens are CD16 and CD56. throcytes, granulocytes, monocytes, platelets, and lympho- CD16 is a receptor for the nonspecific end of antibodies. (See cytes. Each of these lines has specific precursors that originate Chapter 5 for more details.) Because of the presence of CD16, from the pleuripotential stem cells. NK cells are able to make contact with and then lyse any cell Some lymphocyte precursors remain in the marrow to ma- coated with antibodies.10 NK cells are also capable of recog- ture and become NK and B cells. B cells received their name nizing any foreign cell and represent the first line of defense because they were originally found to mature in birds in an against virally infected cells and tumor cells.19 organ called the bursa of Fabricius, which is similar to the ap- Although NK cells have traditionally been considered part pendix in humans. After searching for such an organ in hu- of the innate immune system because they can respond to a mans, it was discovered that B-cell maturation takes place variety of antigens, it appears that they also have the capability within the bone marrow itself. Thus, the naming of these cells to develop memory to specific antigens in a similar manner to was appropriate. Other lymphocyte precursors go to the thy- T cells.19 Normally, NK cells have a half-life of 7 to 10 days, mus and develop into T cells, so named because of where they but new evidence suggests that they are able to survive for a mature.7 Immature T cells appear in the fetus as early as longer time because they can generate highly specific memory 8 weeks in the gestational period.21 Thus, differentiation of cells.19,20 Thus, they play an important role as a transitional lymphocytes appears to take place very early in fetal develop- cell bridging the innate and the adaptive immune response ment and is essential to acquisition of immunocompetence by against pathogens.17 the time the infant is born. Thymus Organs of the Immune System T cells develop their identifying characteristics in the thymus, which is a small, flat, bilobed organ found in the thorax, or Just as the cells of the immune system have diverse functions, chest cavity, right below the thyroid gland and overlying the so, too, do key organs that are involved in the development of heart. In humans, the thymus reaches a weight of 30 to 40 g the immune response. The bone marrow and thymus are con- by puberty and then gradually shrinks in size.22 It was first sidered the primary lymphoid organs where maturation of thought that the thymus produces enough virgin T lympho- B lymphocytes and T lymphocytes takes place, respectively. The cytes early in life to seed the entire immune system, making secondary organs provide a location where contact with foreign the organ unnecessary later on. However, it now appears that 4466_Ch01_001-015 30/08/16 5:01 PM Page 9 Chapter 1 Introduction to Immunity and the Immune System 9 lymphoid tissue (CALT), and MALT in the respiratory, gastroin- testinal, and urogenital tracts. It is within these secondary Adenoids organs that the main contact with foreign antigens takes place. Tonsils Lymphocyte circulation between the secondary organs is com- plex and is regulated by different cell surface adhesion mole- Thymus cules and by cytokines. Lymph nodes Heart Each lymphocyte spends most of its life span in solid tissue, Lungs entering the circulation only periodically to go from one sec- ondary organ to another. Lymphocytes in these organs travel Liver through the tissue and return to the bloodstream by way of the Spleen thoracic duct. The thoracic duct is the largest lymphatic vessel in the body. It collects most of the body’s lymph fluid and emp- Small intestine ties it into the left subclavian vein. The majority of circulating Peyer’s patches lymphocytes are T cells.5 Continuous recirculation increases the likelihood of a T lymphocyte coming into contact with the Bone marrow specific antigen with which it can react. Lymphocytes are segregated within the secondary organs according to their particular functions. T lymphocytes are effector cells that serve a regulatory role, whereas B lymphocytes produce antibodies. It is in the secondary organs that contact Tissue with foreign antigens is most likely to take place. lymphatics Lymphopoiesis, or multiplication of lymphocytes, occurs in the secondary lymphoid tissue and is strictly dependent on antigenic stimulation. Formation of lymphocytes in the bone marrow, however, is antigen-independent, meaning that lym- phocytes are constantly being produced without the presence of specific antigens. Most naïve or resting lymphocytes die within a few days after leaving the primary lymphoid organs unless activated by the presence of a specific foreign antigen. Antigen activation gives rise to long-lived memory cells and shorter-lived effector cells that are responsible for the genera- tion of the immune response. Spleen FIGURE 1–9 Sites of lymphoreticular tissue. Primary organs The spleen, the largest secondary lymphoid organ, has a length include the bone marrow and the thymus. Secondary organs are of approximately 12 cm and weighs 150 g in the adult. It is lo- distributed throughout the body and include the spleen, lymph cated in the upper-left quadrant of the abdomen just below the nodes, and mucosal-associated lymphoid tissue (MALT). The diaphragm and is surrounded by a thin connective tissue cap- spleen filters antigens in the blood, whereas the lymphatic system sule. The organ can be characterized as a large discriminating filters fluid from the tissues. filter as it removes old and damaged cells and foreign antigens from the blood. although the thymus diminishes in size as humans age, it is Splenic tissue can be divided into two main types: red pulp still capable of producing T lymphocytes, although at a dimin- and white pulp. The red pulp makes up more than one-half of ished rate.22,23 the total volume and its function is to destroy old red blood Each lobe of the thymus is divided into smaller lobules filled cells (RBCs). Blood flows from the arterioles into the red pulp with epithelial cells that play a central role in the differentiation and then exits by way of the splenic vein. The white pulp com- process. Maturation of T cells takes place over a 3-week period prises approximately 20% of the total weight of the spleen and as cells filter through the thymic cortex to the medulla. Differ- contains the lymphoid tissue, which is arranged around arteri- ent surface antigens are expressed as T cells mature. In this oles in a periarteriolar lymphoid sheath (PALS) (Fig. 1–10). manner, a repertoire of T cells is created to protect the body This sheath contains mainly T cells. Attached to the sheath are from foreign invaders. Mature T lymphocytes are then released primary follicles, which contain B cells that are not yet stim- from the medulla. ulated by antigens. Surrounding the PALS is a marginal zone containing dendritic cells that trap antigens. Lymphocytes enter and leave this area by means of the many capillary branches Secondary Lymphoid Organs that connect to the arterioles. The spleen receives a blood Once lymphocytes mature in the primary organs, they are re- volume of approximately 350 mL/minute, which allows lym- leased and make their way to secondary lymphoid organs, phocytes and macrophages to constantly survey for infectious which include the spleen, lymph nodes, cutaneous-associated agents or other foreign matter.22 4466_Ch01_001-015 30/08/16 5:01 PM Page 10 10 SECTION 1 Nature of the Immune System Lymphocytes and any foreign antigens present enter nodes via afferent lymphatic vessels. Numerous lymphocytes also enter the nodes from the bloodstream by means of specialized venules called high endothelial venules, which are located in the paracortical areas of the node tissues.24 The outermost layer, the cortex, contains macrophages and aggregations of B cells in primary follicles similar to those found in the spleen. These are the mature, resting B cells that have not yet been exposed to antigens. Specialized cells called follicular dendritic cells are also located here. These cells exhibit a large number of recep- Capsule tors for antibodies and help to capture antigens to present to T and B cells. Central Secondary follicles consist of antigen-stimulated prolifer- artery ating B cells. The interior of a secondary follicle is known as the Germinal germinal center because it is here that transformation of the center B cells takes place. When exposed to an antigen, plasma cells (Fig. 1–12), which actively secrete antibodies, and memory Periarteriolar cells, which are just a step away from forming plasma cells, are lymphoid formed. Thus, the lymph nodes provide an ideal environment sheath (PALS) for the generation of B-cell memory. (T cells) T lymphocytes are mainly localized in the paracortex, the region between the follicles and the medulla. T lymphocytes Trabecular are in close proximity to APCs called interdigitating cells. The vein medulla is less densely populated than the cortex but con- tains some T cells (in addition to B cells), macrophages, and Sinuses in red pulp numerous plasma cells. Primary follicle Particulate antigens are removed from the fluid as it travels Marginal across the node from cortex to medulla. Fluid and lymphocytes (B cells) zone exit by way of the efferent lymph vessels. Such vessels form a FIGURE 1–10 Cross-section of the spleen showing organization of larger duct that eventually connects with the thoracic duct and the lymphoid tissue. T cells surround arterioles in the PALS. B cells the venous system. As a result, lymphocytes are able to recir- are just beyond in follicles. When stimulated by antigens, the B cells culate continuously between lymph nodes and the peripheral form germinal centers. All of the lymphoid tissue is referred to as the white pulp. blood. If contact with an antigen takes place, lymphocyte traffic shuts down. Lymphocytes able to respond to a particular antigen Lymph Nodes proliferate in the node. Accumulation of lymphocytes and other Lymph nodes serve as central collecting points for lymph fluid cells causes the lymph nodes to become enlarged, a condition from adjacent tissues. Lymph fluid is a filtrate of the blood and known as lymphadenopathy. As lymphocyte traffic resumes, re- arises from passage of water and low-molecular-weight solutes circulation of expanded numbers of lymphocytes occurs. out of blood vessel walls and into the interstitial spaces be- Other Secondary Organs tween cells. Some of this interstitial fluid returns to the blood- Additional areas of lymphoid tissue include the mucosal- stream through venules, but a portion flows through the tissues associated tissue known as MALT. MALT is found in the gas- and is eventually collected in thin-walled vessels known as lym- trointestinal, respiratory, and urogenital tracts. Some examples phatic vessels. Lymph nodes are located along lymphatic ducts include the tonsils; appendix; and Peyer’s patches, a specialized and are especially numerous near joints and where the arms type of MALT located at the lower ileum of the intestinal tract. and legs join the body (see Fig. 1–9). These mucosal surfaces represent some of the main ports of Filtration of interstitial fluid from around cells in the tissues entry for foreign antigens, and thus, numerous macrophages is an important function of these organs because it allows con- and lymphocytes are localized here. tact between lymphocytes and foreign antigens from the tissues The skin is considered the largest organ in the body and the to take place. Whereas the spleen helps to protect us from for- epidermis contains a number of intraepidermal lymphocytes. eign antigens in the blood, the lymph nodes provide the ideal Most of these are T cells, which are uniquely positioned to environment for contact with foreign antigens that have pene- combat any antigens that enter through the skin. In addition, trated into the tissues. The lymph fluid flows slowly through monocytes, macrophages, and dendritic cells are found here. spaces called sinuses, which are lined with macrophages, cre- The collective term for these cells is the cutaneous-associated ating an ideal location where phagocytosis can take place. The lymphoid tissue, or CALT. node tissue is organized into an outer cortex, a paracortex, and All of these secondary organs function as potential sites for an inner medulla (Fig. 1–11). contact with foreign antigens and they increase the probability 4466_Ch01_001-015 30/08/16 5:01 PM Page 11 Chapter 1 Introduction to Immunity and the Immune System 11 Afferent lymphatic Germinal Capsule center Cortex (B-cell area) Subcapsular sinus Afferent Afferent lymphatic lymphatic FIGURE 1–11 Structure of a lymph node. A lymph node is surrounded by a tough outer capsule. Right underneath is the subcapsular Primary sinus, where lymph fluid drains from afferent follicle Sinusoid lymphatic vessels. The outer cortex contains collections of B cells in primary follicles. When stimulated by antigens, secondary Secondary follicle with follicles are formed. T cells are found in germinal center the paracortical area. Fluid drains slowly Medulla through sinusoids to the medullary region Paracortex (T-cell area) and out the efferent lymphatic vessel to Efferent the thoracic duct. lymphatic SUMMARY Immunology has its roots in the study of immunity—the condition of being resistant to disease. Jenner performed the first vaccination against smallpox by using cowpox. Louis Pasteur is considered the father of immunology for his use of attenuated vaccines. Metchnikoff was the first to observe phagocytosis—meaning cells that eat cells. Immunity has two branches. Innate immunity is the ability of the body to resist infection through means of normally present nonspecific body functions. Adaptive immunity is characterized by specificity, memory, and dependence FIGURE 1–12 A typical plasma cell. (From Harmening D. Clinical upon lymphocytes. Hematology and Fundamentals of Hemostasis. 5th ed. Philadelphia, All blood cells arise in the bone marrow from hematopoi- PA: F.A. Davis 2009. Fig. 1–47.) etic stem cells. The five principal types of leukocytes are neutrophils, of an immune response. Within each of these secondary eosinophils, basophils, monocytes, and lymphocytes. organs, T and B cells are segregated and perform specialized Tissue cells involved in immunity include mast functions. B cells differentiate into memory cells and plasma cells, dendritic cells, and macrophages that arise from cells and are responsible for humoral immunity or antibody monocytes. formation. T cells play a role in cell-mediated immunity; as Cells that are involved in the innate immune response and such, they produce sensitized lymphocytes that secrete cytokines. are actively phagocytic include neutrophils, monocytes, Both cell-mediated immunity and humoral immunity are part macrophages, and dendritic cells. of the overall adaptive immune response. 4466_Ch01_001-015 30/08/16 5:01 PM Page 12 12 SECTION 1 Nature of the Immune System Lymphocytes are the key cells involved in the adaptive Natural killer (NK) cells are types of lymphocyte that immune response. arise from a lymphocyte precursor but do not develop CD stands for clusters of differentiation, which are types in the thymus. They can kill virally infected or cancerous of proteins found on cell surfaces that can be used for target cells without previous exposure to them. identification of specific cells and cell stages. The bone marrow and the thymus are considered primary B cells are a type of lymphocyte that develop in the bone lymphoid organs. B cells remain in the bone marrow to marrow and are capable of secreting antibody when ma- mature, whereas the thymus is where T cells develop their ture. They can be identified by the presence of CD19 and specific characteristics. surface antibody. Secondary lymphoid organs include the spleen, T cells acquire their specificity in the thymus and consist lymph nodes, mucosal-associated lymphoid tissue of two subtypes: CD4+, which are mainly helper or reg- (MALT), and cutaneous-associated lymphoid tissue ulatory T cells, and CD8+, which are cytotoxic T cells. (CALT). Study Guide: Comparison of T, B, and NK Cells T CELLS B CELLS NK CELLS Develop in the thymus Develop in the bone marrow Develop in the bone marrow Found in lymph nodes, Found in bone marrow, spleen, lymph nodes, Found in spleen, liver thoracic duct fluid 10–15% of circulating lymphocyte pool in blood 5–15% of circulating lymphocyte 60–80% of circulating pool in blood lymphocyte pool in blood Adaptive immunity: end Adaptive immunity: end product of activation is Innate immunity: lysis of virally products of activation antibody infected cells and tumor cells; are cytokines production of cytokines Antigens include CD2, Antigens include CD19, CD20, CD21 surface Antigens include CD16, CD56 CD3, CD4, or CD8 antibody Study Guide: Primary and Secondary Lymphoid Organs LYMPHOID ORGAN CATEGORY ORGANS INVOLVED FUNCTION Primary Bone marrow Produces hematopoietic stem cells; maturation of B and NK cells Thymus Maturation of T cells Secondary Spleen Filters blood Lymph nodes Places where contact between T cells, Mucosal associated lymphoid tissue (MALT) antigens, and B cells occur Cutaneous-associated lymphoid tissue (CALT) 4466_Ch01_001-015 30/08/16 5:01 PM Page 13 Chapter 1 Introduction to Immunity and the Immune System 13 Study Guide: Cells of the Immune System CELL TYPE WHERE FOUND FUNCTION Neutrophil 50–75% of circulating First responder to infection, phagocytosis WBCs, also in tissue Eosinophil 1–3% of circulating WBCs Kill parasites, neutralize basophil and mast cell products, regulate mast cells Basophil < 1% of circulating WBCs Induce and maintain allergic reactions, stimulate production of IgE Mast cell Found in skin, connective Antigen presentation to T and B cells; tissue, mucosal epithelium enhancement and suppression of the adaptive immune response Monocyte 4–10% of circulating WBCs Phagocytosis; migrate to tissues to become macrophages Macrophage In lungs, liver, brain, bone, Phagocytosis; kill intracellular parasites; connective tissue, other tumoricidal activity; antigen presentation tissue to T and B cells Dendritic cell In skin, mucous membranes, Most potent phagocytic cell; most effective heart, lungs, liver, kidney, at antigen presentation other tissue Lymphocyte 20–40% of circulating Subtypes are T cells, B cells, and NK cells; WBCs; also found in lymph T cells produce cytokines, B cells produce nodes, spleen, other sec- antibody in adaptive immune response, and ondary lymphoid organs NK cells are involved in innate immunity 4466_Ch01_001-015 30/08/16 5:01 PM Page 14 14 SECTION 1 Nature of the Immune System CASE STUDIES 1. A 13-year-old girl had her ears pierced at a small jewelry 2. You and a friend are discussing the relative merits of store in a mall. Although she was instructed to clean the immunizations. Your friend says that he doesn’t want to area around the earrings with alcohol, she forgot for the get a tetanus booster shot because he has a good immune first 2 days. On the third day she noticed that the area system and his natural defenses will take care of any pos- around one earlobe was red and slightly swollen. sible infection. You have just been studying this subject in your immunology class. Questions a. Which branch of the immune system is likely the Question cause of the symptoms? a. What argument could you make to convince him that b. What type of cell would you expect to see in the tissue? a tetanus booster is a good idea? REVIEW QUESTIONS 1. Which of the following can be attributed to Pasteur? 7. The ability of an individual to resist infection by a. Discovery of opsonins means of normally present body functions is called b. Observation of phagocytosis a. innate immunity. c. First attenuated vaccines b. humoral immunity. d. Theory of humoral immunity c. adaptive immunity. d. cross-immunity. 2. Which WBC is capable of further differentiation in tissues? 8. A cell characterized by a nucleus with two to five a. Neutrophil lobes, a diameter of 10 to 15 μm, and a large number b. Eosinophil of neutral staining granules is identified as a(n) c. Basophil a. eosinophil. d. Monocyte b. monocyte. c. basophil. 3. The cells that Metchnikoff first observed are associated d. neutrophil. with which phenomenon? a. Innate immunity 9. Which of the following is a primary lymphoid organ? b. Adaptive immunity a. Lymph node c. Humoral immunity b. Spleen d. Specific immunity c. Thymus d. MALT 4. Where are all undifferentiated lymphocytes made? a. Bone marrow 10. What type of cells would be found in a primary follicle? b. Spleen a. Unstimulated B cells c. Thymus b. Germinal centers d. Lymph nodes c. Plasma cells d. Memory cells 5. Which of the following statements is true of NK cells? 11. Which of the following is a distinguishing feature a. They rely upon memory for antigen recognition. of B cells? b. They have the same CD groups as B cells. a. Act as helper cells c. They are found mainly in lymph nodes. b. Presence of surface antibody d. They kill target cells without prior exposure to c. Able to kill target cells without prior exposure them. d. Active in phagocytosis 6. Which cell is the most potent phagocytic cell in the 12. Where do lymphocytes mainly come in contact tissue? with antigens? a. Neutrophil a. Secondary lymphoid organs b. Dendritic cell b. Bloodstream c. Eosinophil c. Bone marrow d. Basophil d. Thymus 4466_Ch01_001-015 30/08/16 5:01 PM Page 15 Chapter 1 Introduction to Immunity and the Immune System 15 13. Which of the following is found on the T cell subset 17. Immunity can be defined as known as helpers? a. the study of medicines used to treat diseases. a. CD19 b. a specific population at risk for a disease. b. CD4 c. the condition of being resistant to disease. c. CD8 d. the study of the noncellular portion of the blood. d. CD56 18. A blood cell that has reddish staining granules and is 14. Which of the following statements best characterizes able to kill large parasites describes adaptive immunity? a. basophils. a. Relies on normally present body functions b. monocytes. b. Response is similar for each exposure c. neutrophils. c. Specificity for each individual pathogen d. eosinophils. d. Involves only cellular immunity 19. Which of the following statements best describes a 15. The main function of T cells in the immune response lymph node? is to a. It is considered a primary lymphoid organ. a. produce cytokines that regulate both innate and b. It removes old RBCs. adaptive immunity. c. It collects fluid from the tissues. b. produce antibodies. d. It is where B cells mature. c. participate actively in phagocytosis. d. respond to target cells without prior exposure. 20. Antigenic groups identified by different sets of antibodies reacting in a similar manner to certain 16. Which of the following is a part of humoral immunity? standard cell lines best describes a. Cells involved in phagocytosis a. cytokines. b. Neutralization of toxins by serum b. clusters of differentiation (CD). c. Macrophages and mast cells in the tissue c. neutrophilic granules. d. T and B cells in lymph nodes d. opsonins. 4466_Ch02_016-030 30/08/16 5:00 PM Page 16 2 Nature of Antigens and the Major Histocompatibility Complex Christine Dorresteyn Stevens, EdD, MT(ASCP) LEARNING OUTCOMES CHAPTER OUTLINE After finishing this chapter, you should be able to: FACTORS INFLUENCING THE IMMUNE RESPONSE 1. Define and characterize the nature of immunogens. TRAITS OF IMMUNOGENS 2. Differentiate an immunogen from an antigen. EPITOPES 3. Discuss several biological properties of individuals that influence the nature of the immune response. HAPTENS 4. Describe four important characteristics of immunogens that affect the ADJUVANTS ability to stimulate a host response. RELATIONSHIP OF ANTIGENS 5. Identify the characteristics of a hapten. TO THE HOST 6. Describe how an epitope relates to an immunogen. MAJOR HISTOCOMPATIBILITY COMPLEX 7. Discuss the role of adjuvants. Genes Coding for MHC Molecules 8. Differentiate heterophile antigens from alloantigens and autoantigens. (HLA Antigens) 9. Explain what a haplotype is in regard to inheritance of major Structure of Class I and II MHC histocompatibility complex (MHC) antigens. Molecules 10. Describe the differences in the structure of class I and class II Role of Class I and II Molecules in the molecules. Immune Response 11. Compare the transport of antigen to cellular surfaces by class I and Clinical Significance of MHC class II molecules. SUMMARY 12. Describe the role of transporters associated with antigen processing (TAP) in selecting peptides for binding to class I molecules. CASE STUDY 13. Discuss the differences in the source and types of antigen processed REVIEW QUESTIONS by class I and class II molecules. 14. Explain the clinical significance of the class I and class II molecules. You can go to DavisPlus at davisplus.fadavis.com keyword Stevens for the laboratory exercises that accompany this text. 16 4466_Ch02_016-030 30/08/16 5:00 PM Page 17 Chapter 2 Nature of Antigens and the Major Histocompatibility Complex 17 KEY TERMS Adjuvant Class I MHC (HLA) molecules Haptens Linear epitopes Alleles Class II MHC (HLA) Heteroantigens Major histocompatibility Alloantigens molecules Heterophile antigens complex (MHC) Antigen Conformational epitope Immunogenicity Transporters associated Epitope with antigen processing Antigen presentation Immunogens (TAP1 and TAP2) Autoantigens Haplotype Invariant chain (Ii) Whereas the innate immune system responds nonspecifically to larger quantity required for the response allows the innate im- certain patterns found on pathogens, the adaptive immune mune response to take care of small amounts of pathogens and system is characterized by specific recognition of individual leave the adaptive response for pathogens that are present in pathogens. Lymphocytes are the key cells that are responsible large numbers. Generally, the larger the amount of an immuno- for the specificity, diversity, and memory that characterize adap- gen one is exposed to, the greater the immune response. How- tive immunity. The immune response of lymphocytes is triggered ever, very large amounts can result in T- and B-cell tolerance, by materials called immunogens, which are macromolecules ca- a phenomenon that is not well understood. pable of triggering an adaptive immune response by inducing There are many ways that we come in contact with im- the formation of antibodies or sensitized T cells in an immuno- munogens in nature. How we are exposed to them and where competent host. Immunogens can then specifically react with they get into our bodies determines the actual amount of such antibodies or sensitized T cells. The term antigen refers to immunogen needed to generate an immune response. Such a substance that reacts with an antibody or sensitized T cells but routes include intravenous (into a vein), intradermal (into the may not be able to evoke an immune response in the first place. skin), subcutaneous (beneath the skin), and oral contact. The Thus, all immunogens are antigens, but the converse is not true. route where the immunogen enters the body also determines However, many times the terms are used synonymously and the which cell populations will be involved in the response.1 For distinction between them is not made. In discussing serological example, if an immunogen enters the body via an intravenous reactions or particular names of substances such as blood route, as might occur with a deep puncture wound, the im- groups, the term antigen is still more commonly used; hence, munogen goes directly to the spleen, where a response is both terms are used in this chapter. mounted. On the other hand, if an immunogen enters subcu- One of the most exciting areas of immunological research taneously, such as through a cut or scratch, local lymph nodes focuses on how and why we respond to particular immuno- are involved. gens. This response is actually caused by a combination of fac- Finally, a genetic predisposition may be involved that allows tors: unique biological properties of the individual, the nature individuals to respond to particular immunogens. This predis- of the immunogen itself, genetic coding of major histocom- position is linked to the MHC and to the receptors generated patibility complex (MHC) molecules that must combine with during T- and B-lymphocyte development. The MHC is a sys- an immunogen before T cells are able to respond, and im- tem of genes that code for cell-surface molecules that play an munogen processing and presentation. This chapter focuses important role in antigen recognition. Further details are found on all these areas and discusses future clinical implications of in a later section in this chapter. some recent findings. Traits of Immunogens Factors Influencing In general, immunogenicity—the ability of an immunogen the Immune Response to stimulate a host response—depends on the following char- acteristics: (1) macromolecular size, (2) foreignness, (3) chem- Biological properties of the individual that influence the nature ical composition and molecular complexity, and (4) the ability of the immune response include several factors such as age, to be processed and presented with MHC molecules.1 Usually, overall health, dose, route of inoculation, and genetic capacity. an immunogen must have a molecular weight of at least In general, older individuals are more likely to have a decreased 10,000 to be recognized by the immune system and the most response to antigenic stimulation. At the other end of the age active immunogens typically have a molecular weight of over scale, neonates do not fully respond to immunogens because 100,000 daltons.1 However, there are exceptions because their immune systems are not completely developed. Overall a few substances with a molecular weight of lower than health plays a role because individuals who are malnourished, 1,000 have been known to induce an immune response. For fatigued, or stressed are less likely to mount a successful im- the most part, the rule of thumb is that the greater the molecular mune response. weight, the more potent the molecule is as an immunogen. A significant quantity of an immunogen must be present in Another characteristic that all immunogens share is foreign- order for an adaptive immune response to take place. The ness. The immune system is normally able to distinguish 4466_Ch02_016-030 30/08/16 5:00 PM Page 18 18 SECTION 1 Nature of the Immune System between self and nonself; those substances recognized as non- Levels of self are immunogenic. This ability is acquired as lymphocytes Protein Organization mature in the primary lymphoid organs. Any lymphocyte Amino acids capable of reacting with self-antigen is normally eliminated. Typically, the more distant taxonomically the source of the im- Primary protein structure munogen is from the host, the more successful it is as a stim- ulus. For example, plant protein is a better immunogen for an animal than is material from a related animal. Occasionally, however, autoantibodies, or antibodies to self-antigens, exist. This is the exception rather than the rule; this phenomenon is discussed in Chapter 15. Immunogenicity is also determined by a substance’s chemi- cal composition and molecular complexity. Proteins and poly- Secondary Alpha helix protein structure saccharides are the most effective immunogens. Proteins are powerful immunogens because they are made up of a variety of units known as amino acids. The particular sequential arrange- Pleated sheet ment of amino acids, the primary structure, determines the sec- ondary structure, which is the relative orientation of amino acids within the chain. Chains are usually arranged in an alpha helix or a beta pleated sheet. The tertiary structure embodies Pleated sheet the spatial or three-dimensional orientation of the entire mole- cule and is based on folding of particular regions of the mole- cule; the quaternary structure is based on the association of two Tertiary Alpha helix or more chains into a single polymeric unit (Fig. 2–1). Because protein structure of the variations in subunits, proteins may have an enormous variety of three-dimensional shapes. In contrast, synthetic poly- mers such as nylon or Teflon are made up of a few simple repeating units with no bending or folding within the molecule, which means these materials are nonimmunogenic. For this reason, they are used in making artificial heart valves, elbow replacements, and other medical appliances. Carbohydrates are less immunogenic than protein because they are smaller than proteins and have a limited number of Quaternary sugars available to create their structures. As immunogens, protein structure carbohydrates most often occur in the form of glycolipids or glycoproteins. Many of the blood group antigens are composed of such carbohydrate complexes. For example, the A, B, and H blood group antigens are glycolipids and the Rh and Lewis FIGURE 2–1 Levels of protein organization. antigens are glycoproteins.2,3 Other carbohydrates that are im- portant immunogens are the capsular polysaccharides of bac- teria such as Streptococcus pneumoniae. Pure nucleic acids and Epitopes lipids are not immunogenic by themselves, although a re- sponse can be generated when they are attached to a suitable Although an immunogen must have a molecular weight of carrier molecule.3 This is the case for autoantibodies to DNA at least 10,000, only a small part of the immunogen is actu- that are formed in systemic lupus erythematosus (SLE). These ally recognized in the immune response. This key portion of autoantibodies are actually stimulated by a DNA-protein com- the immunogen is known as the determinant site or epitope. plex rather than by DNA itself. Epitopes are molecular shapes or configurations that are rec- Finally, for a substance to elicit an immune response it must ognized by B or T cells. There is evidence that for proteins, be subject to antigen processing, which involves enzymatic di- epitopes recognized by B cells may consist of as few as 6 to gestion to create small peptides or pieces that can be complexed 15 amino acids.4 Large molecules may have numerous epi- to MHC molecules to present to responsive lymphocytes. If a topes and each one may be capable of triggering specific an- macromolecule cannot be degraded and presented with MHC tibody production or a T-cell response. Epitopes may be molecules, then it would be a poor immunogen. The particular repeating copies or they may have differing specificities. MHC molecules produced also determine responsiveness to in- They may also be sequential or linear epitopes (i.e., amino dividual antigens. Each individual inherits the ability to produce acids following one another on a single chain) or they may a certain limited repertoire of MHC molecules, discussed in the be conformational. A conformational epitope results fr