Armond S. Goldman
Bellur S. Prabhakar
The immune system consists of factors that provide innate and acquired immunity, and has evolved to become more specific, complex, efficient, and regulated. One of the principal functions of the human immune system is to defend against infecting and other foreign agents by distinguishing self from non-self (foreign antigens) and to marshal other protective responses from leukocytes. The immune system, if dysregulated, can react to self antigens to cause autoimmune diseases or fail to defend against infections.
The immune system is organized into discrete compartments to provide the milieu for the development and maintenance of effective immunity. Those two overlapping compartments: the lymphoid and reticuloendothelial systems (RES) house the principal immunologic cells, the leukocytes. Leukocytes derived from pluripotent stem cells in the bone marrow during postnatal life include neutrophils, eosinophils, basophils, monocytes and macrophages, natural killer (NK) cells, and T and B lymphocytes. Hematopoietic and lymphoid precursor cells are derived from pluripotent stem cells. Cells that are specifically committed to each type of leukocyte (colony-forming units) are consequently produced with the assistance of special stimulating factors (e.g. cytokines).
Cells of the immune system intercommunicate by ligand-receptor interactions between cells and/or via secreted molecules called cytokines. Cytokines produced by lymphocytes are termed lymphokines (i.e., interleukins and interferon-gamma) and those produced by monocytes and macrophages are termed monokines.
Cells of the lymphoid system provide highly specific protection against foreign agents and also orchestrate the functions of other parts of the immune system by producing immunoregulatory cytokines. The lymphoid system is divided into 1) central lymphoid organs, the thymus and bone marrow, and 2) peripheral lymphoid organs, lymph nodes, the spleen, and mucosal and submucosal tissues of the alimentary and respiratory tracts. The thymus instructs certain lymphocytes to differentiate into thymus-dependent (T) lymphocytes and selects most of them to die in the thymus (negative selection) and others to exit into the circulation (positive selection). T lymphocytes circulate through the blood, regulate antibody and cellular immunity and help defend against many types of infections. The other classes of lymphocytes, B cells (antibody-forming cells) and natural killer (NK) cells, are thymic-independent and remain principally in peripheral lymphoid organs.
Cells of the RES provide natural immunity against microorganisms by 1) a coupled process of phagocytosis and intracellular killing, 2) recruiting other inflammatory cells through the production of cytokines, and 3) presenting peptide antigens to lymphocytes for the production of antigen-specific immunity. The RES consists of 1) circulating monocytes; 2) resident macrophages in the liver, spleen, lymph nodes, thymus, submucosal tissues of the respiratory and alimentary tracts, bone marrow, and connective tissues; and 3) macrophage-like cells including dendritic cells in lymph nodes, Langerhans cells in skin, and glial cells in the central nervous system.
Leukocytes, the main cells in the immune system, provide either innate or specific adaptive immunity. These cells are derived from myeloid or lymphoid lineage. Myeloid cells include highly phagocytic, motile neutrophils, monocytes, and macrophages that provide a first line of defense against most pathogens. The other myeloid cells, including eosinophils, basophils, and their tissue counterparts, mast cells, are involved in defense against parasites and in the genesis of allergic reactions. In contrast, lymphocytes regulate the action of other leukocytes and generate specific immune responses that prevent chronic or recurrent infections.
Neutrophils: These are one of the major types of cells that are recruited to ingest, kill, and digest pathogens. Neutrophils are the most highly adherent, motile, phagocytic leukocytes and are the first cells recruited to acute inflammatory sites. Each of their functions is dependent upon special proteins, such as the adherence molecule CD11b/CD18, or biochemical pathways, such as the respiratory burst associated with cytochrome b558.
Eosinophils: Eosinophils defend against many types of parasites and participate in common hypersensitivity reactions via cytotoxicity. That cytotoxicity is mediated by large cytoplasmic granules, which contain the eosinophilic basic and cationic proteins.
Basophils: These cells and their tissue counterparts, mast cells, produce cytokines that help defend against parasites and engender allergic inflammation. These cells display high affinity surface membrane receptors for IgE antibodies and have many large cytoplasmic granules, which contain heparin and histamine. When cell-bound IgE antibodies are cross-linked by antigens, the cells degranulate and produce low-molecular weight vasoactive mediators (e.g. histamine) through which they exert their biological effects.
Monocytes/Macrophages: Monocytes and macrophages are involved in phagocytosis and intracellular killing of microorganisms. Macrophages process protein antigens and present peptides to T cells. These monocytes/macrophages are highly adherent, motile and phagocytic; they marshal and regulate other cells of the immune system, such as T lymphocytes; serve as antigen processing-presenting cells; and act as cytotoxic cells when armed with specific IgG antibodies.
Macrophages are differentiated monocytes, which are one of the principal cells found to reside for long periods in the RES. Macrophages may also be recruited to inflammatory sites, and be further activated by exposure to certain cytokines to become more effective in their biologic functions.
These cells provide efficient, specific and long-lasting immunity against microbes and are responsible for acquired immunity. Lymphocytes differentiate into three separate lines: thymic-dependent cells or T lymphocytes that operate in cellular and humoral immunity, B lymphocytes that differentiate into plasma cells to secrete antibodies, and natural killer (NK) cells. T and B lymphocytes are the only lymphoid cells that produce and express specific receptors for antigens.
T Lymphocytes: These cells are involved in the regulation of the immune response and in cell mediated immunity and help B cells to produce antibody (humoral immunity). Mature T cells express antigen-specific T cell receptors (TcR) that are clonally segregated (i.e., one cell lineage-one receptor specificity). Every mature T cell also expresses the CD3 molecule, which is associated with the TcR. In addition mature T cells display one of two accessory molecules, CD4 or CD8. The TcR/CD3 complex recognizes antigens associated with the major histocompatibility complex (MHC) molecules on target cells (e.g. virus-infected cell). The TcR is a transmembrane heterodimer composed of two polypeptide chains (usually, alpha and beta chains). Each chain consists of a constant (C) and a variable (V) region, and are formed by a gene- sorting mechanism similar to that found in antibody formation. The repertoire is generated by combinatorial joining of variable (V), joining (J), and diversity (D) genes, and by N region (nucleotides inserted by the enzyme deoxynucleotidyl-transferase) diversification. Unlike immunoglobulin genes, genes encoding TcR do not undergo somatic mutation. Thus there is no change in the affinity of the TcR during activation, differentiation, and expansion.
T Helper Cells: These cells are the primary regulators of T cell- and B cell-mediated responses. They 1) aid antigen-stimulated subsets of B lymphocytes to proliferate and differentiate toward antibody-producing cells; 2) express the CD4 molecule; 3) recognize foreign antigen complexed with MHC class II molecules on B cells, macrophages or other antigen-presenting cells; and 4) aid effector T lymphocytes in cell-mediated immunity. Currently, it is believed that there are two functional subsets of T helper (Th) cells. Th1 cells aid in the regulation of cellular immunity, and Th2 cells aid B cells to produce certain classes of antibodies (e.g., IgA and IgE). The functions of these subsets of Th cells depend upon the specific types of cytokines that are generated, for example interleukin-2 (IL-2) and interferon-gamma (IFN-gamma) by Th1 cells and IL-4 and IL-10 by Th2 cells.
Cell-mediated immunity (delayed hypersensitivity) plays an important role in defense against many intracellular infections such as Mycobacterium tuberculosis. This inflammatory reaction is initiated by the recognition of specific antigens by Th1 cells. Consequently, lymphokines are generated which recruit activated macrophages to eliminate foreign antigens or altered host cells.
T Cytotoxic Cells: These cells are cytotoxic against tumor cells and host cells infected with intracellular pathogens. These cells 1) usually express CD8, 2) destroy infected cells in an antigen-specific manner that is dependent upon the expression of MHC class I molecules.
T Suppressor Cells: These cells suppress the T and B cell responses and express CD8 molecules.
Natural Killer Cells: NK cells are large granular lymphocytes that nonspecifically kill certain types of tumor cells and virus-infected cells. Killing by NK cells is enhanced by cytokines such as IL-2 and IFN-gamma. NK cells are also activated by microorganisms to produce a number of cytokines [(IL-2, IFN-gamma, IFN-alpha, and tumor necrosis factor-alpha (TNF-alpha)]. These circulating large granular lymphocytes do not express CD3, TcR or immunoglobulin, but display surface receptors (CD16) for the Fc fragment of IgG antibodies.
B Lymphocytes: These cells differentiate into plasma cells to secrete antibodies and are involved in processing proteins and presenting the resultant peptide antigen fragments in the context of MHC molecule to T cells. The genesis of µ and delta chain-positive, mature B cells from pre-B cells is antigen-independent. Pre-B cells in the bone marrow undergo gene rearrangement for IgM heavy (H) chains and consequently express those proteins in the cytoplasm (the µ chain), but no immunoglobulin light (L) chains. B cell development is characterized by recombinations of immunoglobulin H and L chain genes and expression of specific surface monomeric IgM molecules. At this stage of development, B cells are highly susceptible to the induction of tolerance. Once these cells acquire IgD molecules on their surface, they become mature B cells that are able to differentiate after exposure to antigen into antibody-producing plasma cells.
The activation of B cells into antibody producing/secreting cells (plasma cells) is antigen-dependent. Once specific antigen binds to surface Ig molecule, the B cells differentiate into plasma cells that produce and secrete antibodies of the same antigen-binding specificity. If B cells also interact with Th cells, they proliferate and switch the isotype (class) of immunoglobulin that is produced, while retaining the same antigen-binding specificity. This occurs as a result of recombination of the same Ig VDJ genes (the variable region of the Ig) with a different constant (C) region gene such as IgG. In the case of protein antigens, Th2 cells are thought to be required for switching from IgM to IgG, IgA, or IgE isotypes.
IgM is therefore the principal antibody produced during a primary immunization. This primary antibody response is manifested by serum IgM antibodies as early as 3-5 days after the first exposure to an immunogen (immunizing antigen), peaks in 10 days, and persists for some weeks. Secondary or anamnestic antibody responses following repeated exposures to the same antigen appear more rapidly, are of longer duration, have higher affinity, and principally are IgG molecules.
When antibodies bind to antigens, they may 1) neutralize pathogenic features of antigens such as their toxins, 2) facilitate their ingestion by phagocytic cells (opsonization), 3) fix to and activate complement molecules to produce opsonins and chemoattractants (vide infra), or 4) participate in antibody-dependent cellular cytotoxicity (ADCC).
In addition to antibody formation, B cells also process and present protein antigens. After the antigen is internalized it is digested into fragments, some of which are complexed with MHC class II molecules and then presented on the cell surface to CD4+ T cells.
Immunoglobulins (antibodies) are globular glycoproteins found in body fluids or on B cells where they act as antigen receptors. These molecules are either expressed on the surface of B cells or are secreted by terminally differentiated cells from this lineage (plasma cells) into the circulation or external secretions. An immunoglobulin molecule is a symmetrical multi-chain peptide consisting of two identical H chains and two identical L chains. Each chain is divided into a V region that is responsible for specific antigen binding and a C region that carries out other functions such as the binding of IgG to complement or leukocytes. These antibody molecules are formed as a result of the assembly of separate germ-line genes for the V, J, and C regions of the H and L chains of the final immunoglobulin molecule. This combinatorial mechanism is responsible for the great diversity of antibody molecules.
There are five major isotypes (classes) of immunoglobulins (IgG, IgA, IgM, IgD, and IgE). These isotypes are distinguished by differences in the C regions of H chains of each immunoglobulin isotype (gamma, alpha, µ, delta, and epsilon, respectively). These differences are responsible for the particular functions of immunoglobulin classes.
The specific receptor for antigen on T lymphocytes, the TcR, is a heterodimeric protein with motifs that are similar to immunoglobulin molecules, but whose structure is encoded by a different set of V, J, D, and C genes. Moreover, T cells consist of two subsets carrying different receptors, that have been designated alpha/beta and gamma/delta. The T cell receptors act as specific antigen recognition molecules. Unlike antibody molecules, the TcR molecules cannot recognize soluble antigens. In contrast, they recognize protein antigens that have been processed and presented as peptides on the surface of antigen-presenting cells in the context of MHC class I or MHC class II molecules (vide infra).
These genes encode for cell surface molecules that are involved in the genesis and regulation of specific immune responses to T-cell dependent antigens and in tissue transplantation. They principally encode cell surface protein molecules that bind antigenic peptides, which are recognized by T cells.
The MHC is a cluster of ~ 40-50 genes located on chromosome 6. These genes belong to the super-immunoglobulin gene family. There are three classes of these molecules. MHC class I molecules are found on all nucleated somatic cells and aid in presenting endogenously synthesized antigens, whereas MHC class II molecules are found principally on antigen processing/presenting cells (i.e., macrophages, B cells) and are involved in presenting processed exogenous protein antigens. The MHC class III region contains a heterogeneous group of genes that encode for some components of the complement system, heat shock proteins, tumor necrosis factor-alpha, and tumor necrosis factor-beta.
The presentation of antigen in the context of MHC molecules is essential for T cell recognition of peptide antigens. However, interactions between the MHC-bound peptide and TcR and the MHC class I or class II molecules with CD8 or CD4, respectively, is not sufficient to activate T cells. Other ligands on antigen presenting cells and their receptors (co-receptors/co-stimulators) on T cells are required to complete the process of T cell activation.
Immunologic tolerance (unresponsiveness) normally prevents reactions against self-antigens; if immunologic tolerance is broken, autoimmune reactions may occur. Much of the development of tolerance occurs in the thymus by the elimination (clonal deletion) or inactivation (clonal anergy) of self-reactive clones of T cells. Other mechanisms of tolerance occur extrathymically and include activation of antigen-specific T suppressor cells and clonal deletion, which results in the elimination of self-reactive B cells or T cells, and clonal anergy.
Tolerance may be broken because of a genetic predisposition to immune dysregulation, altered self-antigens, exposure to microbial antigens that cross-react with self-antigens, or exposure to a self-antigen that is normally not revealed to the immune system (e.g., an antigen in the eye). When tolerance against self-antigens is broken, autoimmunity is produced, which could result in an autoimmune disease.
The complement system consists of inactive circulating glycoproteins that can be sequentially activated by antigen-antibody (IgG or IgM) complexes or bacterial products to enhance inflammation or to attack cellular membranes. The system consists of the classical and alternative pathways that converge to activate the membrane attack complex. After activation, opsonic, chemoattractant, or cytotoxic fragments are produced.
Natural (innate) and acquired defenses are marshalled to combat infecting agents. The first line of defense includes the skin, mucous membranes, protective inhibitors, and IgA antibodies produced at mucosal sites. The second line of defense consists of local factors and cells that are activated or recruited to the site of microbial invasion. These include: 1) the coagulation system, 2) the fibrinolytic system, 3) vasoactive peptides, 4) the complement system, 5) resident macrophages, 6) recruited inflammatory leukocytes, and 7) cytokines. The third line of defense includes the expansion of populations of antigen-specific B cells and T cells, the production of systemic antibodies, and the activation of T cells. Successful defense is followed by a clearance of opsonized pathogens by the RES and tissue repair.
Excessive or otherwise inappropriate immune responses to infecting agents may lead to disease. Examples of such excessive immunologic responses that can be protective or cause disease include: 1) circulating antigen-antibody (immune) complexes of microbial antigens bound to IgM or IgG antibodies, 2) antibodies to microorganisms that cross-react with self-antigens, 3) vasoactive compounds from the complement system and from the metabolism of arachidonic acid, 4) excessive production of proinflammatory cytokines, 5) delayed hypersensitivity reactions, and 6) cytotoxic T cells directed against the infected host cells.
The immune system undergoes an orderly development during the prenatal and postnatal periods. Mature T and B cells appear in the fetus, but are not activated until the infant is exposed to immunogens. Memory T cells are not present during early infancy and the antibody repertoire is not fully established for many months. IgM is the first type of antibody produced postnatally. IgG antibodies to protein antigens are formed in early infancy, but IgG antibodies to polysaccharides do not appear until 2 - 2.5 years of age. There are also developmental delays in the production of several cytokines such as the interferons.
Maternal immune factors are transmitted to the fetus via the placenta and to the young infant by mammary gland secretions. These transferred maternal factors compensate for developmental delays in the production of those immune factors by the recipient fetus/infant. Developmental delay in the production of IgG is overcome by transfer of maternal antibodies of that same isotype via the placenta. Other immune factors (whose production is developmentally delayed), such as secretory IgA, lactoferrin, and lysozyme; leukocytes; anti-inflammatory agents; and immunomodulating agents are provided by mammary gland secretions via human milk. These factors are not as well represented in non-human milk. Therefore, the breast-fed infant is less at risk for gastrointestinal and respiratory infections and for inflammatory disorders including common allergic diseases.
Immune deficiencies are genetic or acquired and result in an increased susceptibility to certain infections, the types of which depend upon the exact nature of the defect.
Genetic Defects: X-linked agammaglobulinemia is a genetic defect in a B cell progenitor kinase that is essential for B cell development. Consequently, few B cells and only low levels of antibodies are produced. This leads mainly to an increased susceptibility to highly virulent, encapsulated respiratory bacterial infections.
T cell deficiency is the primary problem in severe combined immunodeficiency (deficiencies of B and T cells). Most cases are due to an X-linked recessive defect in the formation of the gamma-chain common to a number of cytokine receptors. Some autosomal recessive types are due to deficiencies in enzymes such as adenosine deaminases in the purine salvage pathway. Patients with these diseases display few T cells, decreased T cell functions, poor antibody formation, and an increased susceptibility to opportunistic infections such as Pneumocystis carinii.
Hereditary defects also occur in neutrophils. For example, a decrease in leukocyte adherence is due to an autosomal recessive defect in the formation of the common CD18-subunit of leukocyte adherence glycoproteins, whereas deficiency in intracellular killing (chronic granulomatous disease) is due to a deficiency in the production of subunits of cytochrome b558 or ancillary proteins necessary for their stabilization. Consequently, reactive oxygen compounds required for intracellular killing are not produced.
Acquired Defects: Protein-energy malnutrition is the leading cause of immunologic deficiency. A second, but important cause of acquired immunodeficiency is the human immunodeficiency virus (HIV) that attacks CD4+ T cells and macrophages. Also, certain other infections depress or destroy parts of the immune system by different mechanisms.
The human defense system consists of factors that provide innate and acquired immunity against microorganisms. The system evolved from primitive but effective defenses found in more ancient animal species. The innate defenses include 1) structural barriers, 2) acids, bases, and other chemical agents produced at various sites, such as mucosal surfaces, and 3) highly phagocytic, motile scavenger cells that have well-developed killing and digestive powers. As a result of the evolutionary process, the mammalian immune system has become more specific, efficient, regulated, and complex. The development of specialized innate and acquired recognition/regulatory proteins (antibodies, cell receptors, and cytokines) expanded the repertoire, and control the magnitude of the protective responses. One of the most important consequences of this evolution is the ability of the immune system to discriminate between self and non-self antigens and maintain a memory of previous encounters with antigens, including those from microorganisms.
The evolutionary changes allowed development of B and T cells which express antigen-specific receptors on their cell surface. These changes permit humans to survive in an environment laden with microbial pathogens and environmental toxins. The pathogenic features of those microorganisms include the ability to 1) enter the body through portals such as the skin, respiratory system, and the alimentary tract; 2) utilize nutrients from those sites; 3) adhere to epithelium; 4) produce virulence factors and toxins; 5) commandeer the replicative machinery of the host's cells; 6) evade the immunologic system; 7) cripple the defenses of the host; and 8) cause autoimmune responses by acting as cross-reactive antigens.
The salient features of the human defense system that evolved to counteract the pathogenic microorganisms and prevent autoimmune problems will be presented in the rest of this chapter.
The production, maturation, and function of cells of the immune system occur to a great extent in two overlapping organ systems, 1) the lymphoid system consisting of lymphocytes and their supporting structures and 2) the RES consisting of macrophages and related mononuclear phagocytes (Fig. 1-1). In postnatal life, bone marrow is the principal source of pluripotent stem cells that produce precursors of cells that operate in host defense (Fig.1-2). The development of each type of leukocyte is precisely controlled and the controls account for the great specificity of the defense system and the fact that untoward immunologic reactions are relatively uncommon.
Figure 1-1. Major organs in the lymphoid and reticuloendothelial systems
Figure 1-2. Production of blood cells from pluripotent stem cells in the bone marrow.
The lymphoid system consists of organs that house 1) T and B cells that are responsible for antigen-specific immunity and 2) NK cells that are innately cytotoxic to tumor cells and cells expressing foreign antigens. The system is divided into a) central lymphoid organs, the thymus and bone marrow, and b) peripheral lymphoid organs including lymph nodes, spleen, and the mucosa/submucosa of the respiratory and alimentary tracts (Fig. 1-1). Lymphocytes are one of the principal leukocytes found in these organs. There are three major types of lymphocytes (T, B, and NK) that have distinctive surface markers and functions (see sections on T cells, B cells, and NK cells (Fig.1-3 and Table 1-1). Furthermore, the T and B cells in the lymph nodes are confined to discrete zones (Fig. 1-4).
Figure 1-3. Principal surface markers of lymphocyte populations. Molecules that serve as receptors are shown in bold type.
Figure 1-4. Lymph node. Discrete B and T cell zones are found.
The second major cellular system, the RES, (Fig. 1-1) harbors macrophages, which are cells that play major roles in 1) defending against many microbial pathogens and 2) generating specific immune responses by processing protein antigens and presenting the resultant peptide antigen fragments in the context of MHC molecules to T cells. The system consists of 1) monocytes in the blood, 2) macrophages in the liver, spleen, lymph nodes, thymus, bone marrow, connective tissues, and submucosal tissues of the respiratory and alimentary tracts, 3) dendritic cells in lymph nodes, 4) Langerhans cells in skin, and 4) glial cells in the central nervous system. Macrophages not only operate in direct defense (phagocytosis and intracellular killing) but also marshal other parts of the immune system, such as T lymphocytes (see section on T lymphocytes) (Table 1-1).
Cells of the immune system have profound immunoregulatory influences on each other. This regulation is mediated in large part by potent bioactive molecules, called cytokines, which may have autocrine, paracrine, or systemic effects. These polypeptides and glycoproteins are produced by diverse types of cells and act on many different types of cells by binding to high affinity receptors on their surfaces. Their functions include 1) activating or attracting specific types of leukocytes, 2) regulating cell division, 3) modulating the production or actions of other cytokines, 4) promoting or abrogating inflammation, 5) directing certain cells to switch the types of proteins that they produce, and 6) influencing the production of cellular or humoral immunity. Those produced principally by lymphocytes have been termed lymphokines and those principally produced by monocytes and macrophages, monokines. Interleukin is also used to designate many of these agents.
A detailed description of the sources, target cells, and principal functions of these agents is beyond the scope of this presentation, but a synopsis of that information is found in Table 1-2 and the specific roles of certain cytokines are discussed in sections of this chapter that will follow.
Ligands, such as surface molecules, on certain cells of the immune system that bind to receptors on other types of cells may activate the cells bearing the receptors and thus modulate immune responses. Interactions between T cell receptors and the processed peptide in the context of the MHC molecule are examples of the importance of such ligand (peptide)-receptor (T cell receptor) interactions in the immune system.