The immune system is designed to attack and destroy a broad spectrum of foreign antigens/pathogens. However, the immune system must be able to distinguish self from nonself, through a process now known as self-tolerance. If this did not occur, then it would be easy to see how the immune system could direct immune cells against self-tissues.13 The body employs many tactics to avoid attacking itself, but when self-tolerance fails this may lead to the development of an autoimmune disease.
Physical and chemical defenses are the most rudimentary form of innate immunity and the first line of defense against invading pathogens. The skin, the largest organ of the body, has the primary role of providing a physical defense. Alterations in the skin, such as burns or abrasions, allow an easy portal of entry for pathogens. The rapid turnover of intestinal cells also limits systemic infection as cells including infected cells are sloughed frequently. Drugs, such as cell-cycle, phase-specific antineoplastic agents, that disrupt the sloughing process, leave the patient at an increased risk for infections. Likewise, the respiratory tract has its forms of physical defense. The mucus coating the epithelial cells serves in part to prevent microorganisms from adhering to cell surfaces, and the cilia lining the epithelium of the lungs help to repel inhaled organisms. The combination of cilia, mucus, and reactive coughing provides a natural barrier to invasion via the respiratory tract. The low pH of the stomach (pH 1-2) is inhospitable to most organisms and is a chemical defense resulting in the death of the microorganism. Other examples of mechanical or chemical defenses include; normal urine flow, lysozymes in tears and saliva, and the normal flora in the throat, the lower GI tract, and the genitourinary tract. Disruption of the normal physical and chemical defense systems through mechanical ventilation, for example, places the host at substantial risk for penetration by a pathogenic organism.14
Phagocytosis and Opsonization
If an infectious pathogen invades and is able to infiltrate through a host’s physical and chemical defense systems, the cells of the innate immune system are activated to halt progression of the infection. These cells are present from birth and use a preexisting, but limited, repertoire of unique receptors to recognize and destroy pathogens. Innate immune cells include subgroups of leukocytes: monocytes/macrophages, neutrophils, basophils, mast cells, and eosinophils. When stimulated by a foreign pathogen, mast cells, and basophils secrete inflammatory mediators. Monocytes/macrophages, neutrophils, mast cells, and eosinophils act as phagocytes. Phagocytes are cells, which recognize, internalize, and degrade the invading pathogens. This process may occur in two ways: opsonin-dependent or opsonin-independent phagocytosis. For opsonin-dependent phagocytosis, opsonins like antibody (eg, IgG), complement (eg, C3b), or lectin (eg, C-reactive protein) coat the infectious pathogen by sticking to conserved structures on the infectious pathogens. Once the pathogen is opsonized, the opsonin (antibody, complement, or lectin) binds to specific the receptors on the phagocyte (Figure e86-2) and activates the phagocytic process. For opsonin-independent phagocytosis, innate leukocytes use pattern recognition receptors (PRRs), which bind to highly conserved structures present on a large number of different microorganisms. PRRs on the phagocytes directly recognize the conserved ligands, also known as Pathogen Associated Molecular Patterns (PAMPs), on the surfaces of infectious pathogens (Table e86-3), leading to the immediate phagocytosis of the pathogen (see Figure e86-2). The PRRs include the macrophage mannose receptor, macrophage scavenger receptor, and members of the toll-like receptor family. Toll-like receptors are a family of PRRs on the cell-surface of innate leukocytes. To date, at least 10 toll-like receptors have been identified in humans. They recognize a broad spectrum of conserved structures ranging from lipopolysaccharide and flagellin on bacteria, to zymosan on yeast, to double-stranded RNA from RNA viruses (see Table e86-3). Binding of the PAMPs to the toll-like receptors (a PRR) allows the phagocyte to recognize and engulf the pathogen. This binding of toll-like receptors (PRRs) to its PAMPs also results in secretion of chemokines, inflammatory cytokines, and antimicrobial peptides and increased expression of costimulatory proteins (eg, B7) and major histocompatibility complex (MHC) molecules by the phagocyte. This leads to the recruitment and activation of antigen-specific lymphocytes.12,15,16
Phagocytosis of bacteria by macrophages, dendritic cells (DCs), and neutrophils. Macrophages, DCs, and neutrophils recognize bacteria opsonized (coated) with antibody or complement (C3b). On the surface of macrophages, DCs, and neutrophils reside receptors for antibody (Fc receptors) and complement (CR1, CR3, and CR4). In addition, these cells may recognize the bacteria by pattern recognition receptors on the surface of macrophages, DCs, and neutrophils. Pattern recognition receptors include toll-like receptors, scavenger receptors, and mannose receptors.
TABLE e86-3Ligands for Pattern Recognition Receptors ||Download (.pdf) TABLE e86-3 Ligands for Pattern Recognition Receptors
|Pathogen Ligand ||Type of Organism |
|Lipoteichoic acid ||Gram-positive organisms |
|Lipopolysaccharide ||Gram-negative organisms |
|Mannose ||Fungi, gram-positive, gram-negative |
|Double-stranded RNA ||RNA viruses |
|Triacyl lipopeptides ||Gram-positive, gram-negative |
|Peptidoglycans ||Gram-positive |
|Bacterial flagella ||Various |
Cells of the Innate Immune System
Neutrophils, eosinophils, and basophils are considered granulocytes because of the presence of numerous cytoplasmic granules in these cells that contain inflammatory mediators or digestive enzymes. Their names are derived from their staining characteristics. For example, neutrophils are named because they stain a neutral pink. Neutrophils comprise most of the total leukocytes in the bloodstream. They are polymorphonuclear cells, which serve as the primary human defense against invasive bacteria. Neutrophils migrate from the bloodstream into infected or inflamed tissue in response to chemotactic factors, such as IL-8 and breakdown products of complement (C3a and C5a). In this migration, a process termed chemotaxis, neutrophils reach the site of inflammation and then recognize (through the PRRs and PAMPs), adhere to, and phagocytose pathogens. Additionally, complement and antibody can bind to specific epitopes on a pathogen (opsonize), and then bind to their corresponding receptors on neutrophils to phagocytize the pathogen. During phagocytosis, the engulfed pathogen is internalized within the phagocyte into a cytoplasmic lysosome. The neutrophil then releases its granular contents into lysosomes to form phagolysosomal granules, which generates the release of oxidative metabolites that destroy the engulfed pathogens.17
Eosinophils are also granulocytic cells involved in innate immunity, and can migrate from the blood into the tissues. They play a less significant role in combating bacterial infections, but eosinophils play a major role against nonphagocytable multicellular pathogens, such as parasites. After activation via high-affinity receptor for IgE (ie, Fcε), eosinophils exocytose their granules causing the release of basic proteins or reactive oxygen species into the microenvironment, causing lysis of the parasite. In addition to Fcε receptors, eosinophils express lower levels of complement receptor 3 and Fcγ for IgG than neutrophils. The high affinity of eosinophils for IgE contributes to their role in the pathogenesis of allergies.18
Macrophages and monocytes are mononuclear cells capable of phagocytosis. Tissue macrophages arise from the migration of monocytes from the bloodstream into the tissues. Macrophages differ from monocytes by possessing an increased number of Fc and complement receptors. Macrophages are found within specific tissues and are often called histiocytes. However, they are most often referred to by specialized names depending on the site where they are found (eg, Kupffer cells in the liver, osteoclasts in the bone, and microglial cells in the CNS).19 The term reticuloendothelial system was commonly used to refer to phagocytic cells of the reticular connective tissue, but the preferred nomenclature is now the mononuclear phagocyte system.
Despite the first description in 1868 of Langerhans cells, a type of dendritic cell (DC) found in the skin, our current understanding of the biologic function of DCs did not develop until the past decade. Before pathogen recognition, most DCs are in an immature/resting state with limited ability to activate T lymphocytes, but they express numerous receptors (eg, Fc receptors of IgG and IgE, macrophage mannose receptor, and toll-like receptors) enabling rapid recognition and phagocytosis of multiple antigens. Following antigen recognition and particle engulfment, DCs become activated and greatly increase their expression of the MHC class II, B7-1/B7-2 (CD80/CD86), CD40, and adhesion molecules. In addition to phagocytosing pathogens in the innate immune system, macrophages, and DCs act as APCs to stimulate the adaptive immune system. Macrophages and DCs perform this function by internalizing the pathogens, digesting them into small peptide fragments, and then combining these antigenic fragments with MHC molecules, which move to the cells surface and present peptides to the T-cell receptor (TCR) on the surface of a T lymphocyte. The recognition of the antigen/MHC complex by the TCR is the first step in the activation of the T lymphocyte (Figure e86-3). B lymphocytes can also act as APCs, which is important to the development of specific antibodies (Figure e86-4).19,20,21
Induction of T-helper type 1 (TH1) response. (1) The APC, in this case a DC, engulfs the pathogen by any of numerous cell surface receptors (Figure e86-2). After phagocytosis of the bacteria by the DC (A), the pathogen is digested into small peptides and become associated with major histocompatibility (MHC) class II within the endosome (B). Finally, the MHC class II plus peptide is expressed on the surface of the DC (C). The activated DC also secretes interleukin (IL)-12. (2) Naïve CD4+ T-lymphocyte activation requires the T-cell receptor (TCR) to recognize the antigenic peptide in association with MHC class II as well as the B7-1 (CD80) binding to CD28. The binding of CD2-CD58 and LFA-1 (CD11a/CD18) allows adherence between the T lymphocyte and DC. Upon activation, the TH1 CD4+ T lymphocyte secretes IL-2 and interferon (IFN)-γ and increases the production and expression of the IL-2 receptor (ICAM, intercellular adhesion molecule).
Induction of T-helper type 2 (TH2) response. (1A) A B lymphocyte recognizes invading bacteria via its surface immunoglobulin (sIg). (1B) The bound bacteria are phagocytosed into an endosome, where the bacteria are broken down into small peptide fragments. (1C) The small peptide fragments are placed within MHC class II molecules and transported to the surface of the B lymphocyte for antigen presentation to a CD4+ T lymphocyte. (2) CD4+ T-lymphocyte recognition requires antigen recognition within the MHC class II peptide groove by the T-cell receptor (TCR) and a secondary signal from B7-2 from the antigen-presenting cell, in this case a B lymphocyte, binding to CD28 on the T lymphocyte. When both signals are delivered, the CD4+ T lymphocyte becomes activated. In the TH2 environment (see the text), the naive CD4+ T lymphocyte develops into a TH2 subtype and secretes interleukin (IL)-4, IL-5, IL-6, IL-10, and IL-13, which promote a TH2 response. (3) In the presence of these cytokines plus antigen binding to the sIg, the B lymphocyte becomes activated. The activated B lymphocyte becomes a plasma cell (4), which produces and secretes immunoglobulin or becomes a memory B lymphocyte (5). A minority of B lymphocytes become memory B lymphocytes.
Mast cells and basophils act primarily by releasing inflammatory mediators. Mast cells are tissue cells predominately associated with IgE-mediated inflammation. They are especially abundant in the skin, lungs, nasal mucosa, and connective tissue. Granules within the mast cells contain large amounts of preformed mediators that include histamine, heparin, and serotonin. Mast cells can also phagocytize, destroy, and present bacterial antigens to T lymphocytes.20 Basophils are similar to mast cells because they contain granules filled with histamine, but they are typically found circulating in the blood and are not found in connective tissue. Like mast cells, basophils also express high-affinity IgE Fc receptors (Fcε). IgE-mediated anaphylaxis (type I hypersensitivity; Chapter e88) is caused by the degranulation and the release of preformed mediators upon stimulation of mast cell and/or basophil by an allergen binding to IgE bound to the Fcε receptor on their cell surface.21
Soluble Mediators of the Innate Immune System
Soluble mediators of innate immunity involve proteins which include the complement system, mannose-binding lectin, antimicrobial peptides, and C-reactive protein (CRP).11 The complement system consists of more than 30 proteins in the plasma and on cell surfaces that play a key role in immune defense. The four major functions of the complement system include: (a) lysis of certain microorganisms and cells; (b) Stimulation of chemotaxis of phagocytic cells; (c) coating or opsonization of foreign pathogens, which allows phagocytosis of the pathogen by leukocytes expressing complement receptors; and (d) clearance of immune complexes. Complement factors (C3a, C5a) also act as chemotactic factors for phagocytic cells.22 Two different pathways stimulate the complement cascade. In the classical pathway, antibody binds to its target antigen and activates the first component of complement (C1), thereby initiating the complement cascade. The alternative complement pathway relies on the inability of microorganisms to clear spontaneously produced C3b, the active form of third complement protein, from their surface. Patients with hereditary deficiencies of complement have recurrent bacterial infections or immune complex disease because C3b plays a central role in opsonizing bacteria and clearing immune complexes.
Both mannan-binding lectin and CRP are acute-phase reactants produced by the liver during the early stages of an infection. They act as opsonins by binding to infectious pathogens that and serve as an intermediate by binding to their respective receptor on phagocytes. Mannan-binding lectin binds to mannose-rich glycoconjugates on microorganisms, while CRP binds to phosphorylcholine on bacterial surfaces.11,22
The chemokine system consists of a group of small polypeptides and their receptors. Chemokines play an essential role in linking the innate and adaptive immune response by orchestrating leukocyte trafficking. Chemokines possess four conserved cysteines. Based on the positions of the cysteines, almost all chemokines fall into one of two categories: (a) CC group in which the conserved cysteines are contiguous or (b) CXC subgroup in which the cysteines are separated by some other amino acid (X). As with all ligand–receptor interactions, a cell can only respond to a chemokine if the cell possesses a receptor that recognizes the chemokine. Chemokine receptors are unique in that they traverse the membrane seven times. CC receptors (CCR) and CXC receptors (CXCR) bind CC ligands (CCL) and CXC ligands (CXCL), respectively (Table e86-4).
TABLE e86-4Common Chemokines ||Download (.pdf) TABLE e86-4 Common Chemokines
|Receptor ||Cell Expression ||Ligand |
|CCR1 ||Immature DC ||MIP-1α, MIP-1β, MCP-2, RANTES |
|CCR3 ||Eosinophils, basophils ||Eotaxin-1, eotaxin-2, eotaxin-3, MCP-4 |
|CCR6 ||Immature DC ||Exodus-1 |
|CCR7 ||Activated DC ||CCL21 (SLC), CCL19 (ELC) |
|CXCR1/2 ||Neutrophils ||IL-8 |
|CXCR3 ||Natural killer cells, activated T lymphocytes ||IP-10 |
Binding of infectious pathogens to PRRs stimulates the release of chemokines such as macrophage inflammatory protein (MIP)-1α, MIP-1β, MIP-3α, and IP-10 from macrophages and DCs embedded in the tissues. These chemokines attract more immature DCs to the site of inflammation/infection. Immature DCs constitutively express CCR1, CCR5, and CCR6. The interaction between PRRs on the DC to the infectious pathogen causes the activation and maturation of the DC. After activation, DCs downregulate the expression of CCR1, CCR5, and CCR6 and upregulate the expression of CCR7. This switch in chemokine-receptor expression results in the antigen-loaded DC leaving the tissue and migrating toward the lymph nodes.23
Naturally occurring antimicrobial peptides include α-defensins, β-defensins, and cathelicidins. These peptides exhibit antibacterial, antifungal, and antiviral activity. Human antimicrobial peptides range in size from 29 to 37 amino acid residues in length. Neutrophils are rich sources of both α- and β-defensins as well as cathelicidins. Other sources of the human antimicrobial peptides include keratinocytes, Paneth cells of the intestinal and genital tracts, and epithelial cells of the pancreas and the kidney. These peptides can be induced at sites of inflammation or can be constitutively produced. The clinical interest in human antimicrobial peptides centers on their broad-spectrum activity and their rapid onset of killing. They are believed to work by disrupting microbial membranes. An active area of research is how these peptides discriminate between microbial and host membranes.24
The body will generally employ both the innate and adaptive immune responses to rapidly kill foreign pathogens.11 The greatest difference between the innate and adaptive immune responses is in specificity and memory, characterized by antigen-specific receptors located on the surface of B- (sIg) and T lymphocytes (TCR).13 The adaptive immune response also secretes cytokines to further amplify the innate immune response. The adaptive immune response can evolve with each subsequent infection whereas the innate response stays the same with each infection. During B- and T-lymphocyte development, an individual B or T lymphocyte rearranges its immunoglobulin and TCR genes, respectively, to produce a unique immunoglobulin or TCR. This DNA rearrangement generates enough B or T lymphocytes to recognize an estimated 1012 and 1010 antigens, respectively.
The adaptive immune response can be divided into two major arms: humoral and cellular responses. The humoral response is so denoted because it was discovered that the factors that provided the immune protection could be found in the “humor” or fluids (eg, serum, plasma, lymph) and generally refers to antibody responses. To generate a good antibody response, T lymphocytes of the T helper cell phenotype are necessary. B lymphocytes activated in this way can differentiate into plasma cells and secrete antibody or they differentiate into memory B cells that are specific for the pathogen that reacted with its sIg.
Cells of the Adaptive Immune System
T lymphocytes constitute the cell-mediated arm of the adaptive system. The immune protection provided by T lymphocytes cannot be transferred by fluids alone. Rather, it is essential to actually have T lymphocytes present, thus the term cell-mediated immunity. T lymphocytes are specially tailored to defend against infections that are intracellular, such as viral infections, whereas B lymphocytes secrete antibodies that can neutralize pathogens prior to their entry into host cells.
The role of the T lymphocyte is to search respond to various pathogens extracellularly (CD4+ T helper cells and MHC Class II), or intracellularly (CD8+ T cytotoxic cells and MHC Class I). T lymphocytes use a specific antigen receptor, TCR, to propagate the immune response. The TCR is comprised of two chains with each chain having a variable and a constant region. The variation of the amino acid sequence within the variable domain of TCR gives the cell its unique antigen specificity. Linked to the TCR is a complex of single chains known as the CD3 complex.11,21
The MHC is a cluster of genes found on chromosome 6 in humans, also known as the human leukocyte antigen (HLA) complex, that are converted to proteins used by the immune system to distinguish self from non-self and provides a so-called immunologic “fingerprint.” The MHC complex is divided into three different classes: I, II, and III. There are 6 MHC Class I genes (A, B, C, E, F, and G) of which only A, B, and C are considered major. These molecules can be found on all nucleated cells within the body as well as on platelets. As such, MHC Class I antigens are not found on mature red blood cells. Molecules encoded by class II MHC genes include DP, DQ, and DR. The expression of these molecules is more restricted and can be found primarily on APCs, such as macrophages, DCs, and B lymphocytes. The class III HLA antigens encode for soluble factors, complement, and tumor necrosis factors (TNFs).25 In order for a CD4+ T lymphocyte to become activated, it must recognize the antigenic peptide in association with MHC class II (see Figures e86-3 and e86-4).
CD8+ T lymphocytes recognize antigenic peptide in association with class I molecules. Class I molecules generally contain endogenous peptides from within the cell, such as those derived from viruses. In contrast, Class II molecules contain exogenous peptides from antigen that has been phagocytosis and digested, such as bacterial peptides (see Figure e86-3). Thus, the MHC Class I and CD8+ T cell interaction is a sensing system by which the immune system is constantly checking the nucleated cells of the body for what is happening inside the cells of your body.25,26 DCs and to a lesser extent macrophages demonstrate the unique capacity to direct exogenous antigens toward MHC class I molecules, a process termed cross-presentation.27 In contrast, the MHC Class II and CD4+ T cell interaction is a sensing system by which the immune system is constantly checking out what is happening outside of our cells.
Naïve T lymphocytes are cells that have not been previously exposed to an antigen specific for their TCR. These cells require 2 signals for activation. The first signal for activation involves the T lymphocyte recognizing both the processed antigen and the MHC molecule complex. The second signal involves the interaction of the B7-1 (CD80) or B7-2 (CD86) molecule on the APC with the CD28 molecule on the surface of the T lymphocyte (see Figure e86-3 and e86-4). Without the second signal, the naïve T lymphocyte becomes anergic or inactive. Memory T lymphocytes are less dependent on the second signal than are naïve T lymphocytes. CD28 is expressed on both resting and activated T lymphocytes. After the two activation signals, a message is sent through the TCR to the CD3 complex into the cell. Then calcium influx occurs, resulting in activation of the T lymphocyte. Activated CD4+ T lymphocytes begin to express the high-affinity interleukin 2 (IL-2) receptor and release multiple soluble factors (eg, IL-2) to stimulate T lymphocytes and other cells of the immune system (see Figure e86-3). Autocrine stimulation by IL-2 leads to the proliferation of the activated T lymphocyte.
In addition to activation pathways, T lymphocytes can also express inhibitory receptors on their cell surface. One example is cytotoxic T-lymphocyte-associated protein 4 (CTLA-4), which also binds B7, and is only expressed on activated T lymphocytes. When B7 binds to CTLA-4 on an activated T lymphocyte, an inhibitory signal is sent to the T lymphocyte, thereby modulating the T-lymphocyte response.28 The exact mechanism by which CTLA-4 binding inhibits T-lymphocyte activity is not fully understood.
Cell surface markers delineate the functional activity of T-lymphocyte populations. All T lymphocytes express the CD3 protein. Typically, T lymphocytes are further divided into helper cells (CD4+), suppressor cells (CD8+), and cytotoxic cells (CD8+). Each of the subclasses appears to play a distinct role in the cell-mediated immune response. Naïve T lymphocytes express CD45RA, a high-molecular-weight isoform of CD45, while memory T lymphocytes express CD45RO, a lower-molecular-weight isoform of CD45.29 The primary role of CD4+ cells is to stimulate other cells in the immune response. Functionally, CD4+ cells can be divided into T-helper type 1 (TH1), T-helper type 2 (TH2), TH17, T follicular helper (THFH), and T-regulatory (Tregs). This functional system was first described in mice. TH1 cells secrete IL-2 and γ-interferon and stimulate CD8+ cytotoxic cells, while TH2 cells secrete IL-4, IL-5, and IL-10 and stimulate B-lymphocyte production of antibody toward extracellular pathogens.30 Multiple factors determine whether a naïve CD4+ T lymphocyte develops into a TH1 or a TH2 cell. The cytokine microenvironment plays an important role in this development. IL-12 secreted by the APCs promotes TH1, whereas IL-4 promotes TH2 development. Other factors that promote TH1 development include B7-1 (CD80), high affinity of the TCR for the antigen, γ-interferon, and α-interferon. Factors that promote TH2 development include B7-2 (CD86), low affinity of the TCR for the antigen, IL-10, and IL-1.31 THFH also promote B-lymphocyte activation and play a crucial role in generation of memory B-lymphocytes which leads to long-lived antibody responses.32 The TH17 subset was discovered because of selective production of IL-17 and plays an important role in immunity in mucosal tissues and in the pathogenesis of multiple inflammatory and autoimmune disorders.33
CD8+ T lymphocytes recognize antigen in association with MHC class I. CD8+ cytotoxic cells are instrumental in killing cells recognized as foreign, such as those that have become infected by a virus. CD8+ cytotoxic T lymphocytes also play an important beneficial role in the eradication of tumor cells, but moreover are responsible for rejection of transplanted organs.21 Classically, a second type of CD8+ T lymphocytes was a suppressor cell. It is clear that some T lymphocytes help suppress the immune responses, but whether this subset is CD8+ is debatable. Emerging evidence is leading away from CD8+ T lymphocytes toward CD4+, CD25+ T lymphocytes in maintaining self-tolerance. The preferred term for these suppressive T lymphocytes is regulatory T lymphocytes.34
Our understanding of how CD8+ T lymphocytes are activated is constantly evolving. The traditional model involves the interaction of a CD8+ T lymphocyte with an APC, typically a DC. A more potent activation of CD8+ T lymphocyte may result from the interaction of an APC, typically a DC, a CD4+ helper lymphocyte (Th1) and a CD8+ T lymphocyte (Figure e86-5A). This model of CD8+ cytotoxic T lymphocyte activation requires the close proximity of two antigen specific T lymphocytes (the CD4+ and the CD8+ T lymphocytes). In addition, CD8+ cytotoxic T-lymphocyte activation can occur in the absence of direct interaction with CD4+ T lymphocytes. Moreover, CD4+ T-lymphocytes can activate (ie, prime) APCs through CD40. This interaction primes the APC to fully activate CD8+ cytotoxic T lymphocytes (Figure e86-5B).35 It is important to remember that the classification of CD4+ lymphocytes as T-helper lymphocytes and CD8+ lymphocytes as T-cytotoxic lymphocytes is not an absolute. Some CD8+ T lymphocytes secrete cytokines similar to a T-helper lymphocyte, and some CD4+ T lymphocytes can act as cytotoxic cells.
In the classic model of CD8+ T-lymphocyte activation (A), CD4+ and CD8+ T lymphocytes recognize antigen on the same DC. In the presence of interleukin (IL)-2 from the activated CD4+ T lymphocyte and the recognition of antigen in association with major histocompatibility complex (MHC) class I, the CD8+ T lymphocyte becomes activated. In the new model (B), activated CD4+ T lymphocytes activate DCs via CD40 ligand binding to CD40. The activated DC then migrates through the tissues to present antigen to CD8+ T lymphocytes. If recognition via the T-cell receptor (TCR) on the CD8+ T lymphocyte occurs, the DC can fully activate the CD8+ T lymphocyte without the presence of CD4+ T lymphocytes.
Unlike neutrophils and macrophages, cytotoxic T lymphocytes are unable to ingest their targets. They destroy target cells by two different mechanisms; the perforin system, and the Fas ligand pathway. After recognition by the cytotoxic T lymphocyte, cytoplasmic granules containing perforins and granzymes are rapidly oriented toward the target cell, and the contents of the granules are released into the intracellular space. Like the membrane attack complex formed after complement activation, perforins form a pore in the target cell membrane. Besides a direct cytotoxic effect on the target cell, the pores produced by perforins allow the granzymes to penetrate into the target cell to induce apoptosis. The second mechanism of cytotoxicity involves the binding of Fas ligand (FasL) on the cytotoxic T lymphocyte to the Fas receptor on the target cell. The FasL is predominately expressed on CD8+ cytotoxic T lymphocytes and natural killer (NK) cells, and its expression increases after activation. When the Fas receptor on the target cell is bound by FasL expressed by the CD8+ cytotoxic T lymphocyte, the target cell receives a very strong signal inducing it to undergo apoptosis (ie, commit suicide).36 After destroying the target cell by either mechanism, the cytotoxic T lymphocyte detaches from the target cell and attacks other targets.37
A B lymphocyte recognizes antigen via its antibody or immunoglobulin (sIg) located on its cell surface (see Figure e86-4). The sIg can recognize an intact pathogen, such as bacteria, and present antigen to T lymphocytes (ie, acting as APC). However, another major function of B lymphocytes is to differentiate into a plasma cell to produce antibody specific for the invading pathogen, a process that first entails activation of the B lymphocyte. The activation of B lymphocytes also requires two steps: (a) recognition of antigen via the sIg; and (b) the presence of B-lymphocyte growth factors (IL-4, 5, and 6) secreted by activated CD4+ T lymphocytes. Once activated, the B lymphocyte becomes a plasma cell, a differentiated cell capable of producing and secreting antibody and then dying. Some activated B lymphocytes do not differentiate into plasma cells, but rather form a pool of memory B cells. The memory B cells will respond to subsequent encounters with the pathogen, generating a quicker and more vigorous response to the pathogen. Some B lymphocytes can become activated without help from T lymphocytes, but these responses are generally weak and do not invoke memory.11,21
NK cells, often referred to as large granular lymphocytes, are defined functionally by their ability to lyse target cells without prior sensitization and without restriction by MHC. NK cells recognize target cells by two mechanisms. First, NK cells express an IgG Fc receptor, CD16, that allows recognition of IgG-coated cells. Second, NK cells express killer-activating and killer-inhibiting receptors. The killer-activating receptors recognize multiple targets on normal cells, but the binding of MHC class I to the killer-inhibitor receptor blocks the release of perforins and granzymes. Therefore, cells (eg, tumor cells, virally infected cells) that downregulate MHC class I expression are susceptible to NK cell cytolysis. NK cells play important roles in the surveillance and the destruction of tumors and virally infected host cells, and in the regulation of hematopoiesis.11,38
The immune system employs several mechanisms to downregulate responses to prevent autoimmune diseases. Many of these mechanisms are directed at T lymphocyte activation. After activation (about 2 days), T-lymphocytes express CTLA-4 (a second ligand for B7 [CD152]). As previously discussed, when CTLA-4 binds B7, T lymphocyte activity is inhibited. Another mechanism of T lymphocyte inhibition is the programmed cell death 1 (PD-1) system. Once a T lymphocyte is activated, it begins to express the PD-1 receptor. The PD-1 receptor is capable of binding two separate ligands, known as PD-L1 and PD-L2. When bound to its ligand, PD-1 inhibits antigen receptor signaling in T lymphocytes, resulting in decreased production of proinflammatory cytokines by the T lymphocyte.39 Interestingly, the same Fas/FasL system used by CD8+ cytotoxic T lymphocytes to destroy their targets is also a mechanism that can be used to inhibit T lymphocytes. Once T lymphocytes become activated, they begin to express Fas receptors on their cell surfaces. If the Fas receptor on a T lymphocyte is bound by FasL, the T lymphocyte receives a signal inducing it to undergo apoptosis. Certain tissues, such as the testis, retina, and some types of cancer cells use the Fas system to protect themselves from harmful immune responses. These tissues constitutively express the FasL, which protects them from activated T-lymphocytes.40 Another means of modulating T lymphocyte responses is through a functional subset of CD4+ lymphocytes: Tregs. Tregs are antigen specific and require contact between the Tregs and the target lymphocyte in order to exert their inhibitory effect. Tregs can downregulate T-lymphocyte responses by secreting transforming growth factor-β and IL-10.34
Soluble Mediators of the Adaptive Immune Response
When binding of a specific antigen to the surface immunoglobulin receptor of B lymphocytes occurs, the B lymphocyte matures into a plasma cell and produces large quantities of antibody that have the ability to bind to the inciting antigen. The secreted antibodies may be of 5 different isotypes: IgA, IgD, IgE, IgG, and IgM. On primary exposure to a given pathogen, the plasma cell will secrete IgM, followed by an eventual switch to predominately IgG. Upon a second exposure to the same antigen, the memory B lymphocytes will predominately produce IgG. Isotype switching from IgM to IgG, IgA, or IgE is controlled by T lymphocytes.
An antibody or immunoglobulin is a glycoprotein comprised of two different chains, heavy and light (Figure e86-6). The basic structure of every immunoglobulin consists of four peptide chains: two identical heavy chains and two identical light chains held together by disulfide bonds. The basic structure of the antibody is a Y-shaped figure. Each arm of the Y is formed by the linkage of the end of the light chain to its heavy chain partner. These arms contain the portions described as the fragments of antigen binding (Fab fragments). The stem of the Y contains the heavy chains, which comprise the fragment crystallizable (Fc fragment) portion of the antibody. It is within the Fc portion that complement is activated once the antibody has bound its target. Likewise, it is the Fc portion of the antibody that is recognized by Fc receptors on the surface of phagocytes (see Figure e86-2). The amino acid composition of the same isotype is homogenous except in the variable regions of the light (VL) and heavy chains (VH). The variation in amino acid composition of the variable region gives the antibody its unique specificity (see Figure e86-6).
Schematic diagram of the structure of the IgG molecule. IgG molecule consists of 2 heavy (H) and 2 light (L) chains covalently linked by disulfide bonds. Each chain is composed of variable (V) and constant (C) regions. A light chain consists of 1 variable (VL) and 1 constant (CL) region. Heavy chains consist of 1 variable (VH) and 3 or 4 constant (CH) regions, depending on the isotype. The variable regions (VL and VH) compose the antigen-binding region of the IgG molecule, or fragment antigen binding (Fab). The constant regions provide the structure to the IgG molecule as well as binding the first component of complement (CH2) and binding to Fc receptors via the Fc portion of the molecule (CH3).
IgG, the most prevalent of the antibody classes, comprises about 80% of serum immunoglobulins. IgG is usually the second isotype of antibody to be produced in an initial humoral immune response. IgG is the only isotype of antibody that can cross the placenta. Therefore, early maternal humoral protection of neonates is primarily due to maternal IgG that has crossed the placenta in utero.
Four different subclasses of IgG have been described: IgG1, IgG2, IgG3, and IgG4. These subclasses differ slightly in their constant amino acid sequences. IgG1 constitutes the majority (60%) of the subclasses. It appears that different subclasses recognize different types of antigen. IgG1 and IgG3 are principally responsible for recognition of protein antigens, while IgG2 and IgG4 commonly bind to carbohydrate antigens. Another difference in the subclasses is the ability to activate complement with IgG3 and IgG1 being the most efficient, while IgG4 is unable to activate the complement system.
IgM can be found on the surface of B-lymphocytes (sIg) as a monomeric Y-shaped structure. In contrast, secreted IgM is a pentamer in which five of the monomers are joined together by a joining chain (J-chain). IgM is the first class of antibody to be produced on initial exposure to an antigen. Because the pentameric form of IgM has no Fc portions exposed, phagocytic cells cannot bind pathogens opsonized by IgM. However, IgM is an excellent activator of the complement cascade by the classic pathway.
IgA is found primarily in the fluid secretions of the body: tears, saliva, nasal fluids; and also in the GI, genitourinary, and respiratory tracts. IgA functions by preventing pathogens from adhering to and infecting the epithelial cells at these sites. IgA is also secreted in a nursing mother’s breast milk as well as are IgG and IgM but in lower concentrations. In bodily secretions, IgA is in a dimeric form in which a J-chain and a secretory chain hold two monomers together. The dimeric form is resistant to proteolysis in mucosal secretions.
IgD is the least understood isotype. IgD is found on the surface of B-lymphocytes at different stages of maturation and may be involved in the differentiation of these cells. The main function of circulating IgD has not yet been determined. However, mice treated with exogenous anti-IgD antibody display a marked increase in immunoreactivity and secretion of all types of immunoglobulins and several T-cell specific cytokines. High levels of anti-IgD autoantibodies of various subtypes have also been observed in most autoimmune diseases with frequencies of more than 50%, suggesting that IgD may play an important role in the etiology of these diseases.41
IgE is the least common of the serum antibody isotypes. Most of the IgE in the body is bound to the IgE Fc receptors on mast cells. When the IgE on the surface of mast cells binds antigen, it causes the release of various inflammatory substances (eg, histamine) from the mast cell. The overall effect is the stimulation of inflammation. The major function of IgE antibody is to eliminate parasites, but because developed countries of the world have few if any parasites, the response has appeared to shift and it now plays an important role in allergies. Hay fever is an example of allergic reactions primarily due to antigen binding to IgE.
Cytokines are soluble factors released or secreted by cells. These proteins affect the activity of other cells (paracrine) or the secreting cell itself (autocrine). For example, activated CD4+ T lymphocytes secrete IL-2, which further activates the secreting cells, CD8+ T lymphocytes and NK cells. Research has shown that many cytokines have a broad spectrum of effects dependent on their concentration, the presence of other factors, and the target cell (Table e86-5). New cytokine families and their roles in disease processes are being discovered daily. Cytokines provide communication between the divisions of the immune system. Cytokines produced from APCs generally promote chemotaxis of other cells and induce a state of inflammation.38 Cytokines can also prevent activation or response of immunologic cells. For example, IL-10 is an anti-inflammatory cytokine that is produced in the respiratory tract to prevent IgE synthesis and activation of eosinophils when exposed to benign inhaled particles.38 Cytokines do not act alone in vivo, but in combination with other cytokines. For example, activated CD4+ T lymphocytes secrete both IL-2 and interferon-γ, which are synergistic in activating NK cells. As shown in Tables e86-1 and e86-5, cytokines are broadly classified as regulatory or hematopoietic growth factors.21,42,43,44,45,46 This classification does not describe all their activities. For example, GM-CSF released by activated T lymphocytes not only acts as a hematopoietic growth factor, but it also activates circulating granulocytes and APCs to phagocytize foreign pathogens.
TABLE e86-5Regulatory Cytokines ||Download (.pdf) TABLE e86-5 Regulatory Cytokines
|Cytokines ||Sources ||Principal Effects |
|IL-1 ||Macrophages, fibroblasts, endothelial cells ||Activation of T- and B-lymphocytes, hematopoietic growth factor, and induction of inflammatory events |
|IL-2 ||CD4+ T-lymphocytes (TH1 subset) ||Activation of T-lymphocytes, B-lymphocytes, and NK cells |
|IL-4 ||CD4+ T-lymphocytes (TH2 subset), mast cells, basophils, eosinophils ||B- and T-lymphocytes growth factor, activation of macrophages, promotes IgE production, proliferation of bone marrow precursors |
|IL-5 ||CD4+ T-lymphocytes (TH2 subset), mast cells ||Activation of B-lymphocytes and eosinophils, promotes IgE production |
|IL-6 ||CD4+ T-lymphocytes (TH2 subset), macrophages, mast cells, fibroblasts ||T- and B-lymphocytes growth factor, hematopoietic growth factor, augments inflammation |
|IL-8 ||T-lymphocytes, monocytes, endothelial cells, fibroblasts ||Neutrophil, basophil, and T-lymphocytes chemotaxis |
|IL-10 ||T- and B-lymphocytes, macrophages ||Cytokine synthesis inhibitory factor, growth of mast cells |
|IL-12 ||Macrophages, neutrophils, dendritic cells ||Induce TH1 cells, ↑ NK cell activity, ↑ generation of cytotoxic T-lymphocytes |
|IL-13 ||Activated T-lymphocytes ||Proliferation of B-lymphocytes, suppression of proinflammatory cytokines, directs IgE isotype switching |
|IL-14 ||T-lymphocytes ||Induces B-lymphocytes proliferation, inhibits secretion of Igs |
|IL-15 ||Macrophages, fibroblasts, dendritic cells, epithelial cells ||T-lymphocytes proliferation and activation of NK cells |
|IL-16 ||CD8+ T-lymphocytes, epithelial cells ||Chemoattractant for CD4+ T-lymphocytes and eosinophils; stimulation of secondary cytokine secretion from and proliferation of CD4+ T-lymphocytes |
|IL-17 ||CD4+ T-lymphocytes (TH17 subset) ||Proinflammatory cytokine that promotes the neutrophil expansion and accumulation in the tissues |
|IL-18 ||Macrophages ||Induces γ-interferon production |
|IL-28 and 29a ||Antigen presenting cells, but proposed that all nucleated cells may produce ||Alternative to α/β interferons to provide immunity against viral infections by inhibiting viral replication |
|IL-31 ||Activated T-lymphocytes ||Involved in the recruitment of PMNs, monocytes, and T-cells to the site of inflammation |
|IL-32 ||NK-cells, T-lymphocytes, epithelial cells ||Induces proinflammatory cytokines including TNF-α and IL-8 |
|IL-35 ||CD4+ T-lymphocytes (Treg subset) ||T-cell suppression |
|TNF-α ||Macrophages, NK cells, T-lymphocytes, B-lymphocytes, mast cells ||Activation of neutrophils, endothelial cells, lymphocytes and liver cells to produce acute phase proteins |
|TNF-β ||T-lymphocytes ||Tumoricidal |
|IFN-α ||Monocytes, other cells ||Antiviral, activation of NK cells and macrophages, upregulation MHC class I |
|IFN-γ ||T-lymphocytes, NK cells ||Activation of macrophages, NK cells, upregulation of MHC class I and II |
The division of the immune system into the two functional groups does not imply that the divisions do not interact. In order to generate a vigorous immune response, both soluble mediators (eg, complement, antibody, and cytokines) and cells (eg, neutrophils, macrophages, DCs, T lymphocytes, and B lymphocytes) are needed. The innate system will usually respond first. DCs, macrophages, and neutrophils in the tissues will recognize pathogen via surface receptors (see Figure e86-2). In order to amplify the immune response, the APCs will present antigen to CD4+ T lymphocytes (see Figures e86-3 and e86-4). The activated CD4+ T lymphocytes will then secrete cytokines to activate B lymphocytes, CD8+ T lymphocytes, NK cells, macrophages, and neutrophils. The next section of the chapter discusses the evaluation of the immune system.