Chapter 186. Overview of the Immune System

The principal function of the human immune system is to provide protection from the myriad potential pathogens that inhabit the natural world.1 An initial line of defense consists of mechanical barriers such as the skin and ciliated, mucus-covered membranes. Once a microorganism succeeds in breaching one of these barriers, it is typically engaged by an intricate and tightly regulated network of cells and soluble proteins that function collectively to eliminate it or render it harmless, without causing undue injury to the body. The preformed elements of this network provide a rapid response. Over several days to weeks, a more potent and specific immune response develops against unique microbial molecules.

To effectively carry out its role in host defense, the immune system must perform two additional tasks. First, in order to avoid forming “autoimmune” responses against the body itself, there must be mechanisms to maintain self-tolerance. Second, since an overly exuberant or prolonged reaction could be detrimental, there must be ways to “turn off” an immune response once a potential threat has been contained.

The first progenitor cells of the immune system are found in the yolk sac at a gestational age of approximately 3 weeks. These pluripotent hematopoietic stem cells (Fig. 186-1) seed the liver at 5 weeks, and hematopoiesis begins at 6 weeks of gestation. By the 12th week, hematopoiesis has shifted to the bone marrow.2 Delivery of growth, differentiation, and migration signaling to and from immune cells is mediated in large part by soluble proteins termed cytokines and chemokines and their respective receptors.3 Lymphoid precursor cells either mature locally into bone marrow-derived B cells or are exported to the thymus where they further mature into T cells. Other cells of hematopoietic origin with important immune roles include natural killer (NK) cells and antigen-presenting cells. Each cell type expresses a unique collection of cell surface molecules, the composition of which changes during development and during an immune response. These surface molecules have been classified by the World Health Organization as “cluster of differentiation” (CD), and include coreceptors, adhesion molecules, homing molecules, and cytokine receptors (eTable 186.1). Identifying and characterizing immune cells based on their cluster of differentiation surface phenotype has become an indispensable tool of the modern immunologist.

The thymus is formed from the third brachial cleft and the third/fourth brachial pouch, which may contribute both endoderm and ectoderm. Embryonic tissue from these locations moves caudally to fuse in the midline, forming the two thymic lobes. This process is under the control of several transcription factors. TBX1 is a T-box family transcription factor expressed in head mesenchyme and developing pharyngeal arches and pouches. Mutations in or genetic deletions encompassing the TBX1 gene may result in DeGeorge syndrome, which in its complete form includes thymic agenesis.4

Mature T and B lymphocytes migrate to secondary lymphoid organs, the lymph nodes, mucous membrane-associated tissue, and spleen at a gestational age of 10 to 12 weeks. It is in the secondary lymphoid tissues that most mature lymphocytes reside and where specific responses are generated. Responses to bloodborne antigens occur in the spleen, which is connected to the systemic circulation. Lymphocytes in lymph nodes respond to antigens delivered from the skin and via the lymphatics. The tonsil, Peyer patches, and lamina propria of the gut contain lymphocytes that respond to antigens entering through enteric mucosal surfaces, whereas lymphocyte responses to antigen encountered in the lung commence within the pulmonary parenchyma and in peribronchial lymph nodes.

Innate defense mechanisms include mechanical barriers; mononuclear cells, including macrophages, phagocytes, and natural killer cells; as well proteins with microbicidal properties. These mechanisms share in common the capacity to recognize and rapidly engage microorganisms.5

Cellular innate immune responses may be triggered by pattern recognition receptors that recognize pathogen-associated molecular patterns (PAMPs). The latter are molecular motifs expressed by microbes but not by mammalian cells (Fig. 186-2). Among them, Toll-like receptors, named for their structural homology to a receptor found in Drosophila, appear to be particularly important. They are expressed by dendritic cells, endothelial cells, neutrophils, and lymphocytes. Toll-like receptors that recognize bacterial motifs are expressed primarily on the cell surface, whereas those that recognize viral molecules are expressed intracellularly. Toll-like receptors binding to a microbial pathogen-associated molecular pattern stimulates intracellular biochemical pathways, which in turn leads to production of proinflammatory and antiviral cytokines, recruitment of inflammatory cells, and modulation of immune cell function.7 As discussed further in Chapter 187, mutations in molecules involved in Toll-like receptor signaling are associated with hereditary susceptibility to infection by specific pathogens.

Neutrophils are the first cells to respond to most pathogens and their rapid appearance depends on a carefully orchestrated series of events. TNF-α or leukotrienes are produced locally in response to bacteria. The response of sentinel cells is critical for the early response. Initial inflammatory mediators drive increased expression of adhesion molecules on the surface of the vascular endothelium near the infection (Fig. 186-3). This causes the circulating neutrophils to slow and roll along the endothelial surface. Additional signals from chemokines are required to further activate adhesion of the neutrophil to the endothelium. The neutrophil will then undergo diapedesis across the endothelium into the tissue space.2 As the neutrophil is motile, it can migrate toward the bacteria. The primary endogenous chemotactic signal is a complement cleavage product, C5a. Neutrophils also have receptors for a formylated peptide, f-Met-Leu-Phe, which is a pattern seen only in bacteria. The neutrophil migrates up the gradient of chemotactic molecules and finally arrives at the site of bacteria. Here, ingestion occurs. This too is a highly regulated process. Neutrophils have receptors that recognize oligosaccharides typical of bacteria and can directly ingest bacteria via those receptors. Ingestion is much more efficient if the bacteria are coated with antibodies (from a previous infection) or the complement protein C3b. Once ingested, the bacterium is assaulted by reactive oxygen species produced by the NADPH complex on the membrane of the phagosome and the release of toxic granule contents into the phagosome.3 The combined effects are nearly uniformly fatal to bacteria. Neutrophils play little role in defense against viruses but do contribute to the clearance of bacteria, fungi, mycobacteria, and certain parasites.

Macrophages and monocytes are related by lineage. Traditionally, the word monocyte refers to the cell in the bloodstream and the word macrophage refers to the cell in the tissue. There are functional differences that accompany the transformation from monocyte to macrophage, but the simplest distinction to be made is location of the cell. Tissue macrophages represent one of the main sentinel cells in the body; others are mast cells and dendritic cells. These cell types are all characterized by high expression of receptors for (PAMPs) discussed above (Fig. 186-2). When the sentinel cells bind to a pathogen, it releases TNF-α, IL-12, and IL-6 within a few hours. When the pathogen is a virus, a different program is followed, leading to production of type I interferons (the α and β interferons). For bacteria, the cytokines drive increased expression of adhesion molecules on the vascular endothelium to recruit neutrophils and enhance phagocytosis. Certain pathogens are resistant to neutrophil killing, such as mycobacteria and fungi. These are killed by activated macrophages: Although macrophages have limited ability to kill pathogens in their resting state, activation by γ-interferon and TNF-α dramatically improves intracellular killing (Fig. 186-4). Macrophages can become further activated in a poorly understood process, then aggregate and form granulomas.

Natural killer (NK) cells are a subset of lymphocytes distinguished by their large size, granular cytoplasm, and expression of distinct surface molecules. They selectively identify and kill virally infected cells and tumor cells, using cytotoxic mechanisms to lyse target cells. Following contact with a target cell, organelles are oriented toward the target by a calcium-dependent mechanism. This is followed by a release of granules containing granzyme A, perforin, and TGF-β. NK cells are identified by their expression of CD16 (FcγRIII) and CD56 (NCAM-1) on the cell surface and comprise 10% to 15% of circulating lymphocytes. Mature NK cells can be found in the spleen, lungs, and liver. During the first trimester of pregnancy, they are also found in the placental decidua. NK cells use an array of stimulatory and inhibitory receptors to modulate their cytolytic functions. The NK cell group 2, member D, is an important activating receptor that recognizes molecules expressed by cells during stress, such as that following viral infection or malignant transformation.8

In addition to their effector functions, NK cells also have an important role in attenuating inflammatory responses. Genetically determined and acquired NK cell defects are associated with a severe autoinflammatory condition termed hemophagocytic lymphohistiocytosis (see Chapter 463).

Other components of the innate immune system include antimicrobial peptides expressed on mucosal surfaces, termed defensins, as well as the serum proteins C-reactive protein and mannose binding lectin. The complement system is discussed further in Chapter 189.

The adaptive immune system is distinguished by two features. First, in contrast to cells of the innate immune system that recognize molecular motifs common to many microbes, adaptive immune responses are directed against unique peptides, termed antigens. This property is called immune specificity.1 Second, initial exposure of the adaptive immune system to an antigenic stimulus leads to immune sensitization. Subsequent re-exposure to the same antigen results in a more rapid and vigorous immune response, a process referred to as immunologic memory. Adaptive immune responses are broadly divided into cellular and humoral immune responses, mediated mainly by T and B lymphocytes, respectively.

T cells recognize antigens through a dimeric T-cell receptor. In 95% of thymocytes, the T-cell receptor consists of one α chain and one β chain, and in approximately 5% it comprises γ chain and one Δ chain. Each chain has both constant and variable gene segments. The variable regions are formed through combinatorial joining of gene segments and other imprecise mechanisms of diversity associated with DNA rearrangement. It is estimated that there are 1016 different T-cell antigen receptors. It stands to reason that with so many different receptors, some of them could potentially bind self-proteins rather than infectious agents. The functional immune system must eliminate those cells with self-reactive receptors from the repertoire, thereby avoiding autoimmunity, while maintaining those cells capable of recognizing foreign proteins. This central task of the developing immune system is referred to as education and occurs mainly in the thymus.

Multipotent stem cells enter the thymus as early as the eighth week of gestation. During their migration through the thymic cortex to the medulla, a number of maturational events occur. Under the influence of thymic epithelium, a portion of thymocytes commit to the T-cell lineage and undergo, in succession, rearrangement of the genes that will eventually form their respective T-cell receptors. These recombinational events are dependent on functional recombinase-associated genes, without which T-cell development is arrested or severely impaired. Maturation of T cells continues in the thymic cortex through a series of developmental stages (Fig. 186-5). Expression of low levels of CD4 combined with rearrangement of T-cell receptor α genes marks commitment to the T-cell lineage. Next, developing thymocytes express low levels of CD8. This is followed by rearrangement of the T-cell receptor β locus, leading to expression of a pre–T-cell receptor on the cell surface. Cells that fail to express the pre–T-cell receptor die by apoptosis, a process termed β selection. Ultimately, the pre–T-cell receptor α chain is replaced by the T-cell receptor α chain, generating an antigen-specific T-cell receptor. These intermediate-stage to late-stage progenitors express low levels of the αβ T-cell receptor and both CD4 and CD8. A parallel line of development occurs for T-cell receptor γδ cells.9

Maturation of these CD4, CD8 “double positive” cells beyond this stage depends on the affinity of the interaction between the unique T-cell receptor on each double-positive thymocyte and self-peptide/major histocompatibility complex (MHC) expressed on the surface of thymic epithelial cells. If the interaction between T-cell receptor and peptide/MHC complex is “weak,” then the cells die by neglect, having failed to receive a positive signal. If the interaction is too “strong,” then the cells die by activation-induced cell death (negative selection). This later process is the primary mechanism by which self-reactive T cells are eliminated from the repertoire. Last, if the interaction is “appropriate,” then those thymocytes receive a survival signal and mature to become either CD4 or CD8 single-positive cells (positive selection).

Further processes important in establishing central immunologic tolerance occur in the thymic medulla.10 Medullary thymic epithelial cells express self-antigens that are ordinarily expressed in peripheral tissues. Single-positive thymocytes that recognize these antigens are deleted, further increasing the stringency of central tolerance. Self-antigen expression in the thymus is at least partly controlled by the AIRE gene. AIRE mutations may lead to impaired central tolerance, resulting in an autoimmune syndrome termed autoimmune polyendocrinopathy-candidiasis-ectodermal dystrophy (APCED). Within the thymic medulla, possibly within specialized structures known as Hassal’s corpuscles, it is believed that self-antigens are presented by dendritic cells to a subset of single-positive thymocytes, which are induced to differentiate into natural regulatory T cells (Treg cells). Treg cells express the transcription factor FoxP3 and are exported to the periphery, where they suppress immune responses and contribute to “peripheral” immune tolerance. Genetic defects in FOXP3 result in another autoimmune syndrome, known as immune polyendocrinopathy-enteropathy, X-linked.12

Peripheral T-cell expansion and the development of memory T cells are the final phases of T-cell development. T-cell receptors do not recognize antigen directly. Instead, T cells containing the αβ T-cell receptor recognize microbial peptides displayed by a self-structure, an MHC molecule. Intracellular peptides are displayed by Class I MHC molecules and are recognized by CD8+ T cells, whereas extracellular antigen is ingested by specialized cells termed antigen-presenting cells (described below), proteolytically cleaved, and displayed to CD4+ T cells by class II MHC molecules. When the T cell’s antigen-specific receptor is engaged by a specific peptide bound to an MHC molecule expressed on the surface of the antigen-presenting cell, an “immunologic synapse” is initiated, involving intracellular signal transduction and reorganization of signaling molecules in the membrane. Activation of naïve T cells requires a second signal other than that provided through the T-cell receptor. The best-characterized costimulatory molecule on the surface of naïve T cells is CD28. CD28 binds to B7-1 and B7-2 on the surface of antigen-presenting cells and increases intracellular tyrosine phosphorylation. T cells possessing the γδ form of the T-cell receptor do not recognize peptide in the context of MHC. Rather, they recognize heat-shock proteins and may respond to antigen presented in other ways at mucosal surfaces.

Upon activation, naïve T cells secrete the T-cell growth factor IL-2. Over a period of days, and under the control of unique transcription factors, those T cells that are CD4+ may further differentiate and acquire specialized effector functions, including characteristic cytokine secretion profiles. These T-cell subsets include Th1, Th2, Th17, and induced Treg cells, each with a specific immune function (Fig. 186-6).13 Imbalances in T-cell subsets have been associated with various human diseases. Most T cells generated by a primary immune response undergo apoptotic cell death. However, a small number of cells, proportional to the initial antigenic load and clonal burst size, survive by unknown mechanisms and differentiate into memory cells. In general, memory cells allow a more rapid, more potent, and more enduring secondary response that is the basis for vaccine-induced immunity. Among T cells in the peripheral blood, the memory phenotype increases with age.

B cells are the progenitors of antibody-secreting plasma cells, which provide the main form of protection against many extracellular pathogens. Similar to T cells, B-cell development begins in the fetal liver (and possibly the omentum) before a gestational age of 7 weeks, subsequently shifting to the bone marrow. B-cell development involves a complex antigen-independent program that can be divided into stages, as shown in Figure 186-7.1 In the early pro–B-cell stage, lymphoid progenitor cells commit to the B-cell lineage, but Ig gene rearrangement has not yet occurred. Subsequently, pro-B cells undergo genetic recombinational events, partly dependent on recombinase-associated genes and terminal deoxynucleotidyltransferase, leading to generation of Ig μ heavy chain protein.

The μ chain is subsequently expressed on the cell surface together with surrogate light chains, which are invariant and are formed in the absence of DNA rearrangement, forming the pre–B-cell receptor. Expression of the pre–B-cell receptor promotes B-cell differentiation toward the pre–B-cell stage. Signals delivered through this receptor are transduced in part by a molecule termed Bruton tyrosine kinase. Genetic mutations in components of the pre–B-cell receptor or Bruton tyrosine kinase result in B-cell maturational arrest and are responsible for hereditary forms of agammaglobulinemia.14 If pre–B-cell receptor function is intact, light-chain rearrangement takes place, and the newly rearranged immunoglobulin light-chain protein supplants the surrogate light-chain components to generate an antigen-specific B-cell receptor. The B-cell receptor contains the complete IgM molecule, marking the transition to the immature B-cell stage of development.

Negative selection occurs at the immature B-cell stage by two mechanisms. Some cells with autoreactive B-cell receptors are deleted in the bone marrow by apoptosis. Alternatively, the specificity of autoreactive cells can be modified by further gene rearrangement, a process termed receptor editing.15 Surviving immature B cells are exported as transitional B cells to the spleen. Depending on the avidity of B-cell receptor for autoantigen, transitional B cells with self-reactive receptors may be eliminated or rendered anergic to further stimulation. Those that survive the selection process rapidly differentiate into IgM+, IgD+, B220+ mature B cells, which enter follicular areas, thereby joining the recirculating pool of B lymphocytes.

Mature B cells serve several roles in immunity. First, they function as antigen-presenting cells. Antigen-specific B cells can capture soluble protein antigen at vanishingly low concentrations, process the antigen into small peptide fragments, and present them to T cells in the context of MHC class II molecules. Second, and more importantly, B cells can differentiate into antibody-secreting plasma cells.

Antibodies are composed of two identical heavy chains and two light chains. Initially, all mature B cells express IgM and IgD on their cell surface. However, upon exposure to antigen, and under the influence of cytokines and stimulatory surface interactions with T cells, B cells can undergo isotype switch differentiation. By this process, B cells bearing surface IgG, IgA, and IgE surface receptors are generated, and antigen receptor specificity is preserved. IgM, the first immunoglobulin (Ig) to be generated in a primary immune response, is secreted as a pentamer. IgG is the most abundant Ig in human serum, composing 80% of the total Ig. IgG is divided into four subclasses, based on structural differences. IgA comprises 10% to 15% of serum Ig and contains two subclasses, IgA1 and IgA2. Secreted IgA2 is the major Ig at mucosal surfaces, where it exists as a complex comprising two IgA monomers, a joining chain, and a secretory component. IgE plays a role in immunity to parasites and mediates allergic reactions.

Ig exerts its effects in several ways. Upon binding to bacteria, IgM or IgG molecules undergo conformational changes, enabling them to bind and activate complement, which in turn can lead to perforation of the bacterial membrane. IgG molecules bind to three types of Fcγ receptors located on macrophages, lymphocytes, NK cells, and granulocytes and mediates such functions as antibody-dependent cellular toxicity and transfer of Ig from mother to fetus. In contrast, high-affinity IgE receptors reside mainly on mast cells and basophils. Binding of multivalent antigen to mast cell-bound IgE leads to mast cell activation, thereby initiating an allergic response.

Fetal Ig synthesis occurs at low levels before gestational week 20. Beginning around gestational week 32, maternal IgG is actively transferred across the placenta, such that by term, neonatal IgG ultimately exceeds the maternal concentration by about 10%. IgM, IgA, and IgE cross the placenta in negligible amounts. After birth, neonatal Ig synthesis increases as the passively acquired IgG levels fall. An IgG nadir serum concentration of about 400 mg/dl is reached by 3 to 4 months of age. Synthesis of all Ig continues to increase, reaching adult levels at 1 year for IgM, 5 to 6 years for IgG, and adolescence for IgA. Because normal concentrations change markedly over time, age-normalized reference standards are essential for correctly interpreting the results of clinical laboratory Ig measurements in infants and children.

Although B cells are capable of recognizing antigen on intact proteins, T cells require that the protein be processed into peptide fragments, termed epitopes, which are presented for recognition in the context of MHC molecules. Specialized cells, called antigen-presenting cells, have evolved for this purpose and include B cells themselves, macrophages, and dendritic cells.16

Dendritic cells are hematopoietically derived and distributed throughout the body. Initially, they function as antigen-capturing cells, sampling their surroundings by different mechanisms, including phagocytosis, micropinocytosis, and an array of specialized receptors that recognize IgG, IgE, or mannose residues on the surface of pathogens. After antigen is internalized, dendritic cells may be activated by any of several stimuli, including cytokines, Toll-like receptors, and intracellular pathogen-associated molecular pattern receptors.17 Activation leads to a maturational process that drives them from peripheral tissues to the lymph nodes and spleen. Internalized antigen is targeted to specialized late-endosomal compartments, which degrade proteins to peptides. Ingested, extracellular antigen is presented in the context of MHC class II molecules to CD4+ T cells. Endogenously synthesized viral proteins are also degraded into peptides that are presented in the context of MHC class I to CD8+ T lymphocytes.

As well as presenting antigen, dendritic cells also greatly influence the nature of the ensuing T-cell response. Antigen presentation by a dendritic cell that expresses costimulatory molecules typically results in T-cell activation. In contrast, antigen presentation alone in the absence of costimulation can produce an anergic T-cell response, thereby contributing to antigen-specific immune tolerance. Furthermore, depending on the conditions under which they are activated, dendritic cells can produce cytokines with antiviral properties, such as interferons α and β, or cytokines that direct T cells toward a respective Th1, Th2, or Th17 cell phenotype. Thus, dendritic cells have a central role in initiating and controlling immune responses.

Pathologic host immune responses are often classified into four categories, although it is important to remember that these processes are part of a complex pathogen defense strategy as well.

Immediate or type 1 hypersensitivity is the typical allergic response discussed in Section 14. Preexisting IgE directed against an antigen is bound to mast cells. Engagement of IgE by the antigen leads to immediate degranulation of mast cells with release of histamine as well as a variety of cytokines.31

Type II hypersensitivity refers specifically to autoantibody-mediated tissue damage. The membrane attack complex of the complement cascade primarily harms cells such as erythrocytes, which have no ability to repair membrane damage. Autoantibodies can also lead to tissue damage by facilitating uptake by the reticuloendothelial system. An example of this is idiopathic thrombocytopenia purpura. In other cases, autoantibodies serve as an opsonin for the self-tissue and lead to the recruitment of neutrophils and macrophages, which then induce tissue damage. This type of autoantibody-mediated damage occurs in rheumatic fever, where antibodies to Streptococcus cross-react with myocardial antigens, leading to inflammation and tissue damage.32 Finally, autoantibodies can induce harm by acting as antagonists for a receptor or in some way interfering with receptor function, as is seen in myasthenia gravis.

Immune complex disease or type III hypersensitivity is best exemplified by serum sickness. Originally described in the course of passive immunization to diphtheria, it now most often occurs secondary to drug use (see Chapter 197).33 Classically, approximately 7 to 10 days after beginning a medication, as antibody production begins, the patient develops fever, arthritis, and proteinuria. A vasculitic rash sometimes also may be seen. Skin, joint, and kidney are characterized by small arterial beds with high oncotic pressure, the anatomic region most likely to be involved during serum sickness. This high oncotic pressure leads to deposition of immune complexes in involved areas, in turn activating Fc receptors locally. This then leads to the production of inflammatory cytokines and recruitment of neutrophils and other inflammatory cells. Some manifestations of systemic lupus erythematosus may be mediated through this process. Cryoglobulinemia also leads to an immune complex-like process.

Delayed or type IV hypersensitivity is familiar to clinicians as the basis of the PPD test for tuberculosis. Here, T cells are responsible for the tissue damage. Although a break in T-cell tolerance is hypothesized to underlie many autoimmune diseases, there are few disorders in which the main mechanism of tissue damage is believed to be T-cell driven. Despite the association of many autoimmune diseases with MHC haplotypes, in fact dramatic infiltrates of T cells are seen in relatively few diseases of childhood. Examples include multiple sclerosis, diabetes mellitus, and Crohn disease. In juvenile idiopathic arthritis, the synovial fluid often contains high numbers of neutrophils, a testament to the role of the innate immune system in the signs and symptoms of the disease. In the synovial membrane, the infiltrating cells are largely macrophages and T cells.34 In some cases, infiltrating cells adopt a lymph node-like architecture, including a germinal center. This lymphoid aggregate is believed to require a T-cell contribution, but the exact role of the T cell in this complex process is not known. One theory is that the recognition of antigen by T cells may drive cytokine production, thereby leading to many of the downstream effects.