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 GI tract also provides physical defense. The low pH of the stomach (pH 1 to 2) is inhospitable to most organisms. 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 antineoplastics 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. Other examples of mechanical or nonspecific 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 defense system through mechanical ventilation, for example, places the host at substantial risk for penetration by a pathogenic organism.12
If an infectious pathogen invades and is able to infiltrate through a host’s physical defense system, innate immunity is employed to halt progression of the infection. Innate immunity is present from birth and utilizes a preexisting but limited repertoire of receptors to recognize and destroy pathogens. Innate immune cells include subgroups of leukocytes; specifically, 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, which allow them to recognize, internalize, and destroy invading pathogens. This process may occur in two ways: opsonin-dependent or opsonin-independent phagocytosis. For opsonin-dependent phagocytosis, antibody (e.g., IgG), complement (e.g., C3b), or lectin (e.g., C-reactive protein) coat, or opsonize, the infectious pathogens. Once the pathogen is opsonized, the antibody, complement, or lectin binds to the receptors on the phagocyte (eFig. 21-2) and activates the phagocytic process. For opsonin-independent phagocytosis, innate leukocytes utilize pattern recognition receptors. Pattern recognition receptors recognize highly conserved structures present on a large number of microorganisms. These highly conserved structures are essential for the microorganism’s survival or pathogenicity. The pattern recognition receptors include the macrophage mannose receptor, macrophage scavenger receptor, and members of the toll-like receptor family. Pattern recognition receptors on the phagocytes directly recognize ligands (eTable 21-3) on the surfaces of infectious pathogens leading to immediate phagocytosis of the pathogen (eFig. 21-2). Toll-like receptors are a family of pattern recognition receptors on the cell-surface of innate leukocytes. To date, 11 toll-like receptors have been identified in humans. They recognize a broad spectrum of antigens ranging from lipopolysaccharide and flagellin on bacteria to zymosan on yeast to double-stranded RNA from RNA viruses (eTable 21-3). Binding of the ligand to the toll-like receptors allows the phagocyte to recognize and engulf the pathogen. This binding of toll-like receptors to its ligand also results in the secretion of chemokines, inflammatory cytokines, and antimicrobial peptides as well as the increased expression of costimulatory proteins (e.g., B7) and the MHC proteins by the phagocyte. This leads to the recruitment and activation of antigen-specific lymphocytes.10,13,14 Other pattern recognition receptors that mediate phagocytosis include macrophage receptor with collagenous structure (MARCO), DC-SIGN (dendritic cell-specific intercellular adhesion molecule-3-grabbing nonintegrin), and dectin 1.
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, 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.
eTable 21-3 Ligands for Pattern Recognition Receptors |Favorite Table|Download (.pdf)
eTable 21-3 Ligands for Pattern Recognition Receptors
|Pathogen Ligand||Type of Organism|
|Lipoteichoic acid||Gram-positive organisms|
|Mannose||Fungi, gram-positive, gram-negative|
|Double-stranded RNA||RNA viruses|
|Triacyl lipopeptides||Gram-positive, gram-negative|
Cells of the Innate Immune System
Neutrophils, eosinophils, and basophils are considered granulocytes because of the presence of numerous cytoplasm granules in these cells that contain inflammatory mediators or digestive enzymes. Their names are derived from their staining characteristics; neutrophils are named because they stain a neutral pink. Neutrophils comprise the majority of leukocytes in the bloodstream. They are polymorphonuclear cells, often denoted as PMNs for this reason, 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 C3a and C5a, breakdown products of complement. In this migration, a process termed chemotaxis, neutrophils reach the site of inflammation and then recognize, adhere to, and phagocytose pathogens. Via the complement and antibody receptors located on its surface, neutrophils can recognize and engulf pathogens opsonized with complement or IgG (antibody). During phagocytosis, the engulfed pathogen is internalized within the phagocyte into a cytoplasmic lysosome. The neutrophil then releases its granular contents into lysosome and generates the release of oxidative metabolites that destroy the engulfed pathogens.15 Neutrophils can also recognize pathogens via toll-like receptors.
Eosinophils are also granulocytic cells involved in innate immunity. They exhibit motility and 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 (i.e., 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 allergic disorders (i.e., allergic asthma).16
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 such as the liver, spleen, GI tract, lymph nodes, brain, and others. These specific types of macrophages are often called histiocytes, or referred to by a specialized name depending on the site where they are found (for example, Kupffer cells in the liver, osteoclasts in the bone, and microglial cells in the CNS). The term reticuloendothelial system (RES) was commonly used to refer to macrophages found in reticular connective tissue, but the preferred nomenclature is now the mononuclear phagocyte system.17
Despite the first description in 1868 of Langerhans cells, a type of 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 (e.g., Fc receptors of IgG and IgE, macrophage mannose receptor, and toll-like receptors) enabling rapid antigen recognition. Following antigen recognition and particle engulfment, DCs become activated. This leads to a dramatic increase in their expression of the MHC class II, B7, CD40, and adhesion molecules. DCs then begin to migrate through the tissues toward lymphoid organs (e.g., spleen, lymph nodes) to present antigen to T lymphocytes, causing activation of the adaptive immune system.18
In addition to phagocytosing pathogens, macrophages and DCs act as APCs to stimulate the adaptive (specific) system. Macrophages and DCs internalize the organism, digest it into small peptide fragments, and then combine these antigenic fragments together with MHC proteins. Once the APC has formed the antigen/MHC complex, the APC places the complex on its surface. This surface complex can then be recognized by the 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 (eFig. 21-3). Other cells, B lymphocytes and mast cells, can also act as APCs (eFig. 21-4).17–19
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 (eFig. 21-2). 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 major histocompatibility complex (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.18 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; eChap. 22) is caused by the stimulation of mast cell and/or basophil degranulation and the release of preformed mediators after allergen binds to IgE bound to the Fcε receptor on the surface of mast cells or basophils.19
Soluble Mediators of the Innate Immune System
Soluble mediators of innate immunity include the complement system, mannose-binding lectin, antimicrobial peptides, and C-reactive protein (CRP).9 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) to lyse certain microorganisms and cells, (b) to stimulate the chemotaxis of phagocytic cells, (c) to coat or opsonize foreign pathogens, which allows phagocytosis of the pathogen by leukocytes expressing complement receptors, and (d) to clear immune complexes. Complement factors (C3a, C5a) act as chemotactic factors for phagocytic cells.20 Two different pathways stimulate the complement cascade. In the classic 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 bind to infectious pathogens that prompt the activation of the lectin or minor pathway of the complement system. Mannan-binding lectin binds to mannose-rich glycoconjugates on microorganisms while CRP binds to phosphorylcholine on bacterial surfaces.9,20
Chemokines play an essential role in linking the innate and adaptive immune response by orchestrating leukocyte trafficking. The chemokine system consists of a group of small polypeptides and their receptors. 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 (eTable 21-4).
eTable 21-4 Common Chemokines |Favorite Table|Download (.pdf)
eTable 21-4 Common Chemokines
|CCR1||Immature DC||MIP-1α, MIP-1β, MCP-2, RANTES|
|CCR3||Eosinophils, basophils||Eotaxin-1, eotaxin-2, eotaxin-3, MCP-4|
|CCR7||Activated DC||CCL21 (SLC), CCL19 (ELC)|
|CXCR3||Natural killer cells, activated T lymphocytes||IP-10|
Binding of infectious pathogens to pattern recognition receptors 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 pattern recognition receptors 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.21
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 source 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.22
Adaptive Immune Response: Antigen Recognition
The body will generally employ both the innate and adaptive immune responses to rapidly kill foreign pathogens.9 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 and T lymphocytes.11 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, respectively. This DNA rearrangement generates enough B or T lymphocytes to recognize an estimated 1015 antigens.
The adaptive immune response can be divided into two major arms: humoral and cellular mediated. 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 serum. B lymphocytes comprise the humoral arm. Activated B lymphocytes can differentiate into plasma cells that secrete immunoglobulin or memory B cells specific for each pathogen. T lymphocytes constitute the cell-mediated arm of the adaptive system. The immune protection provided by T lymphocytes cannot be transferred by serum 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.
Adaptive Immune Response: Cells Which Mediate Antigen Recognition
The role of the T lymphocyte is to search and destroy pathogens that infect and replicate intracellularly. When these pathogens enter a cell they are no longer vulnerable to innate host defenses; therefore, it is critical that the T lymphocytes be able to distinguish which cells are infected and which cells are not. T lymphocytes do not recognize intact antigens, such as a bacterial cell wall. T lymphocytes only recognize processed antigens in association with MHC.
The major histocompatibility complex, a cluster of genes found on chromosome 6 in humans, is also known as the human leukocyte antigen (HLA) complex. The MHC is used by the immune system to distinguish self from nonself and provides a so-called immunologic “fingerprint.” The genes from this complex encode for molecules that play a pivotal role in immune recognition and response. The MHC complex is divided into three different classes: I, II, and III. The molecules encoded by class I HLA genes include HLA-A, HLA-B, and HLA-C antigens. These molecules can be found on all nucleated cells within the body, as well as on platelets. Class I antigens are not found on mature red blood cells. Molecules encoded by class II HLA genes include HLA-DP, HLA-DQ, and HLA-DR. The expression of these molecules is more restricted and can be found primarily on cells of the immune system, namely APCs such as macrophages, DCs, and B lymphocytes. The class III HLA antigens encode for soluble factors, complement, and tumor necrosis factors.23 In order for a CD4+ T lymphocyte to become activated, CD4+ T lymphocyte must recognize the antigenic peptide in association with MHC class II (eFigs. 21-3 and 21-4). 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, while class II molecules contain exogenous peptides from antigen that has been phagocytosis and digested, such as bacterial peptides (eFig. 21-3). For it to destroy a virally infected cell, a CD8+ cytotoxic T lymphocyte requires two steps. First, its TCR must recognize the antigenic fragment, such as a viral protein, in association with MHC class I. The second step involves the costimulatory step of B7-CD28 binding. This process is further defined below. Because any cell can become infected, it is advantageous that the CD8+ cytotoxic T lymphocytes recognize the MHC class I molecule that is expressed on all cells except red blood cells. The ability of the MHC class I to present endogenous peptides allows the CD8+ cytotoxic T lymphocytes to constantly screen cells for infections.23,24 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.25
APCs (e.g., macrophages, DCs) engulf the pathogen, digest it, and express its peptide fragments on their cell surface in association with their MHC. 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.9,19
Naïve T lymphocytes are cells that have not been previously exposed to an antigen specific for their TCR. These cells require two 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 (eFigs. 21-3 and 21-4). 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 while CTLA-4, a second ligand for B7 on T lymphocytes, is expressed only on activated T lymphocytes. CTLA-4 binding B7 transduces a negative signal so it plays a role in downregulating a T-lymphocyte response.26 After the two activation signals, a message is sent through the TCR to the CD3 complex into the cell. Then a calcium influx occurs with subsequent activation of the T lymphocyte. Activated CD4+ T lymphocytes begin to express the high-affinity IL-2 receptor and to release multiple soluble factors (e.g., IL-2) to stimulate T lymphocytes and other cells of the immune system (eFig. 21-3). Autocrine stimulation by IL-2 leads to the proliferation of the activated T lymphocyte.
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 CD45.27 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, 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, whereas TH2 cells secrete IL-4, IL-5, and IL-10 and stimulate B-lymphocyte production of antibody toward extracellular pathogens.28 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-1.29 TFH also promote B-lymphocyte activation and play a crucial role in generation of memory B-lymphocytes which leads to long-lived antibody responses.30 TH17 subset were discovered because of selective production of IL-17 and play an important role in immunity in mucosal tissues. They may play a significant role in the pathogenesis of multiple inflammatory and autoimmune disorders.31
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.19 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.32
To fully activate the CD8+ cytotoxic T lymphocyte requires CD4+ T lymphocyte activation, namely the TH1 subset, and its subsequent secretion of IL-2 (eFig. 21-5A). This model of CD8+ cytotoxic T lymphocyte activation requires the close proximity of two-antigen specific T lymphocytes. In addition, some CD8+ cytotoxic T-lymphocyte responses can occur in the absence of CD4+ T lymphocytes. New data suggest that CD4+ T-lymphocytes can activate/prime APCs through CD40. This interaction primes the APC (e.g., DC) to fully activate CD8+ cytotoxic T lymphocytes (eFig. 21-5B).33 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. After destroying that target cell by either mechanism, the cytotoxic T lymphocyte detaches from the target cell and attacks other targets.34
A B lymphocyte recognizes antigen via its antibody or immunoglobulin located on its cell surface (eFig. 21-4). The antibody on the surface can recognize an intact pathogen, such as bacteria, and present antigen to T lymphocytes (i.e., acting as APC). However, the major function of B lymphocytes is to produce antibody to bind to 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 by the surface immunoglobulin 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. A fraction of activated B lymphocytes do not differentiate into plasma cells, but rather form a pool of memory cells. The memory 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.9,19
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. Resting NK cells express the intermediate-affinity IL-2 receptor, CD122. Upon exposure to IL-2, NK cells exhibit greater cytotoxic activity against a wide variety of tumors. 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; however, the binding of MHC class I to the killer-inhibitor receptor blocks release of perforins and granzymes. Therefore, cells (e.g., 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.9,35
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). CTLA-4 (CD152) binding to B7 produces a negative signal to downregulate T-lymphocytes. In inflamed tissues, another mechanism of downregulation predominates, the programmed cell death 1 (PD1) system. In this system of downregulation, the PD1 protein becomes expressed on T-lymphocytes after activation. Inflamed tissues upregulate two ligands for PD1, PD-L1 (programmed cell death 1 ligand; CD274) and PD-L2 (programmed cell death 1 ligand; CD275). Binding of either of these ligands to PD1 results in the downregulation of the T-lymphocytes.26 Another use of the Fas system in addition to a mechanism by which CD8+ T-lymphocytes destroy their targets, the Fas system can provide a mechanism for the downregulation of T-lymphocyte responses. After activation, T-lymphocytes upregulate the cell surface expression of both Fas (CD95) and the ligand for this molecule (FasL). Binding of FasL (either expressed on the surface of a cell or soluble) to CD95 on the T-lymphocyte induces apoptosis (cell death) of the T-lymphocyte. Certain tissues (testis, retina, and some tumors) use the Fas system to protect themselves from harmful immune responses. These tissues constitutively express the FasL, which protects them from activated T-lymphocytes.36 Finally, the last mechanism to downregulate T-lymphocytes responses does not involve surface proteins, but a functional subset of CD4+ lymphocytes, T-regs. Tregs are antigen specific and require contact between the Tregs and the target cells. Tregs downregulate T-lymphocyte responses by secreting transforming growth factor-β and IL-10.32
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 five different isotypes. On primary exposure to the pathogen, the plasma cell will secrete IgM; then, eventually, there is a switch to predominately IgG. On second exposure, 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 (eFig. 21-6). 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 (eFig. 21-2). 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 (eFig. 21-6).
Schematic diagram of the structure of the IgG molecule. IgG molecule consists of two heavy (H) and two light (L) chains covalently linked by disulfide bonds. Each chain is composed of variable (V) and constant (C) regions. A light chain consists of one variable (VL) and one constant (CL) region. Heavy chains consist of one variable (VH) and three or four 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 antibody. 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 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, but IgG4 is unable to activate the complement system.
IgM can be found on the surface of B-lymphocytes 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 >50%, suggesting that IgD may play an important role in the etiology of these diseases.37
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 (e.g., histamine) from the mast cell. The overall effect is the stimulation of inflammation. Asthma and hay fever are two examples 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 activates itself as well as activating CD8+ T lymphocytes and NK cells. Research has shown that many cytokines (eTable 21-5) have a broad spectrum of effects dependent on their concentration, the presence of other factors, and the target cell. 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.35 Monocytes, as previously mentioned, use pattern recognition receptors, enabling the immune system to distinguish pathogenic proteins from nonpathogenic proteins through toll-like receptors stimulating T-lymphocyte activation.35 Cytokines can also prevent activation or response of immunologic cells. For example, IL-10 is an antiinflammatory cytokine that is produced in the respiratory tract to prevent IgE synthesis and activation of eosinophils when exposed to benign inhaled particles.35 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 eTables 21-1 and 21-5, cytokines are broadly classified as regulatory or hematopoietic growth factors.19,38–42 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.
eTable 21-5 Regulatory Cytokines |Favorite Table|Download (.pdf)
eTable 21-5 Regulatory Cytokines
|IL-1||Macrophages, fibroblasts, endothelial cells||Activation of T- and B-lymphocytes, hematopoietic growth factor, and induction of inflammatory events|
|IL-2||CD4+ T-lymphs (TH1 subset)||Activation of T-lymphs, B-lymphs, and NK cells|
|IL-4||CD4+ T-lymphs (TH2 subset), mast cells, basophils, eosinophils||B- and T-lymph growth factor, activation of macrophages, promotes IgE production, proliferation of bone marrow precursors|
|IL-5||CD4+ T-lymphs (TH2 subset), mast cells||Activation of B-lymphs and eosinophils, promotes IgE production|
|IL-6||CD4+ T-lymphs (TH2 subset), macrophages, mast cells, fibroblasts||T- and B-lymph growth factor, hematopoietic growth factor, augments inflammation|
|IL-8||T-lymphs, monocytes, endothelial cells, fibroblasts||Neutrophil, basophil, and T-lymph chemotaxis|
|IL-10||T- and B-lymphs, 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-lymphs|
|IL-13||Activated T-lymphs||Proliferation of B-lymphs, suppression of proinflammatory cytokines, directs IgE isotype switching|
|IL-14||T-lymphs||Induces B-lymph proliferation, inhibits secretion of Igs|
|IL-15||Macrophages, fibroblasts, dendritic cells, epithelial cells||T-lymph proliferation and activation of NK cells|
|IL-16||CD8+ T-lymphs, epithelial cells||Chemoattractant for CD4+ T-lymphs and eosinophils; stimulation of secondary cytokine secretion from and proliferation of CD4+ T-lymphs|
|IL-17||CD4+ T-lymphs (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-lymphs||Involved in the recruitment of PMNs, monocytes, and T-cells to the site of inflammation|
|IL-32||NK-cells, T-lymphs, epithelial cells||Induces proinflammatory cytokines including TNF-α and IL-8|
|IL-35||CD4+ T-lymphs (Treg subset)||T-cell suppression|
|TNF-α||Macrophages, NK cells, T-lymphs, B-lymphs, mast cells||Activation of neutrophils, endothelial cells, lymphs and liver cells to produce acute phase proteins|
|IFN-α||Monocytes, other cells||Antiviral, activation of NK cells and macrophages, upregulation MHC class I|
|IFN-γ||T-lymphs, 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 (e.g., complement, antibody, and cytokines) and cells (e.g., 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 (eFig. 21-2). In order to amplify the immune response, the APCs will present antigen to CD4+ T lymphocytes (eFigs. 21-3 and 21-4). 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.