The adaptive immune system differs from the innate immune response in its discrimination between self and nonself and in the magnitude and diversity of highly specific immune responses possible (Table 2–3). In addition, it has a memory function, which is able to mount an accelerated response if an invader returns. The adaptive system operates in two broad arms—humoral immunity and cell-mediated immunity. Humoral immunity comes from bone marrow-derived B cells and acts through the ability of the antibodies it produces to bind foreign molecules called antigens. Cell-mediated (cellular) immunity is mediated through T cells that mature in the thymus and respond to antigens by directly attacking infected cells or by secreting cytokines to activate other cells. As shown in Figure 2–8, the B-cell and T-cell systems are interactive.
Acquired immune system development. A. Lymphocyte stem cells develop into B- and T-cell precursors that migrate to the bone marrow or thymus, respectively. Mature B and T cells seed secondary lymphoid tissues. B. Lymphocyte receptor binding of antigen activates B and T cells to become effector cells. C. B lymphocytes develop into memory cells and antibody-secreting plasma cells. D. T cells develop into memory cells, helper T cells, and cytotoxic T cells. (Reproduced with permission from Willey JM: Prescott, Harley, & Klein's Microbiology, 7th edition. McGraw-Hill, 2008.)
TABLE 2–3Cells Involved in the Adaptive Immune System ||Download (.pdf) TABLE 2–3 Cells Involved in the Adaptive Immune System
|CELL ||FUNCTION ||SPECIFIC RECEPTORS FOR ANTIGEN ||CHARACTERISTIC CELL SURFACE MARKER ||SPECIAL CHARACTERISTICS |
|B cells ||Production of antibody ||Surface immunoglobulin (IgM monomer) ||Fc and complement C3d receptors; MHCclass II ||Differentiate into plasma cells |
|Helper T lymphocytes (TH) ||Stimulate macrophages, eosinophils, PMNs, IgE production, B cells ||α/β T-cell receptor (TCR) ||CD4+ ||Presented by MHC class II, Three subsets (TH1, TH2, TH17) |
|Cytotoxic T lymphocytes (CTLs) ||Lyse antigen-expressing cells such as virally infected cells or allografts ||α/β TCR ||CD8+ ||Presented by MHC class I |
|Natural killer (NK) cells ||Spontaneous lysis of tumor and infected cells ||Inhibitory; activating ||Fc receptor for IgG ||Recognize MHC class I |
|Macrophages (monocytes) ||Phagocytosis, secretion of cytokines to activate T cells (eg, IL-1) or other accessory cells such as polymorphonuclear neutrophils (PMNs)c ||None, but can be “armed” by antibodies binding to Fc receptors ||Macrophage surface antigens ||Express surface receptors for the activated third component of complement (C3), kill ingested bacteria by oxidative bursts |
|Polymorphonuclear leukocytes (neutrophils, eosinophils) ||Phagocytosis killing ||None, but can be “armed” by antibodies || ||Protective in bacterial and parasitic (eosinophils) infections |
An antigen is any substance (usually foreign) with the ability to stimulate an immune response when presented in an effective fashion. They are usually large structurally complex proteins, polysaccharides, or glycolipids. Each antigen can contain many subregions that are the actual antigenic determinants, or epitopes. These epitopes can consist of separate peptides, carbohydrates, or lipids of the correct size and three-dimensional configuration to fit the combining site of an antibody molecule or a T-cell receptor (TCR) (Figure 2–9). Approximately six amino acids or monosaccharide units provide a correctly sized epitope. Antigens presented by infectious agents typically contain multiple epitopes, including copies of the same epitope. Other small organic molecules that would not ordinarily stimulate an immune response may do so if bound to a larger carrier, such as a protein. These are called haptens, and the specificity of the immune response may be generated for both the hapten and its larger carrier.
Epitopes. Schematic of epitope recognition by an immunoresponsive lymphocyte. Epitope B on the antigen binds to a complementary recognition site on the surface of the immunoresponsive cell. Antigens may have many different epitopes, but an immunoresponsive lymphocyte has receptors of only one specificity. In most cases, epitopes are recognized on the surface of macrophages that have processed the antigen. The receptor for antigens on B cells is the combining site of the surface immunoglobulin.
Antigens stimulate immune response
Epitopes fit to the combining site of T-cell receptors and antibodies
A foreign antigen entering a human host may, by chance, encounter a B cell whose surface antibody is able to bind it. This interaction stimulates the B cell to multiply, differentiate, and produce more surface and soluble antibodies of the same specificity. Eventually, the process leads to production of enough antibody to bind more of the antigen. This mechanism is most likely to operate with antigens such as polysaccharides that have repeating subunits, thus improving the possibility that exposed epitopes are recognized.
B cells multiply and produce antibody
Large, complex antigens such as proteins and viruses must be processed before their epitopes can be effectively recognized by the immune system. This processing takes place in macrophages or specialized epithelial cells found in the skin and lymphoid organs, where they are adjacent to other immunoresponsive cells. The ingested antigen is degraded to peptides of 10 to 20 amino acids that are presented by major histocompatibility molecules on the host cell surface to be recognized by T cells (Figures 2–10, 2–11).
Protein antigens must be processed first
Recognition of Foreignness
Distinguishing between self and nonself is obviously essential to maintaining integrity and homeostasis. The collection of genes that control these functions is called the major histocompatibility complex (MHC), and it codes for molecules present on the surface of almost all human cells. Of interest in infection are MHC class I and II molecules (Figure 2–10). MHC class I molecules are in the membrane of almost all cells, but MHC class II are present only on certain leukocytes such as macrophages, dendritic cells, and some T and B cells.
MHC class I and II molecules. A. The class I molecule is a heterodimer composed of the alpha protein, which is divided into three domains: α1, α2, and α3, and the protein β2 microglobulin. B. The class II molecule is a heterodimer composed of two distinct proteins called alpha and beta. Each is divided into two domains α1, α2, and β1, β2, respectively. (Reproduced with permission from Willey JM: Prescott, Harley, & Klein's Microbiology, 7th edition. McGraw-Hill, 2008.)
MHC gene complex codes surface molecules
MHC II on macrophages, dendritic cells
Both MHC class I and class II participate in antigen processing, but by distinctly different pathways (Figure 2–11). MHC class I molecules bind to products generated in the cytoplasm by a natural process or a viral infection. Viral proteins are digested to peptides in a cytoplasmic structure called the proteasome, and delivered to the endoplasmic reticulum. Here they find the binding site of the class I molecule and are transported to the surface for presentation of the peptide. MHC class II molecules bind to fragments that originally come from outside the cell, but have been taken into the endocytotic vacuole of a phagocyte. After digestion in the phagolysosome, peptide fragments are combined with class II molecules and move to the surface for presentation. The presented MHC class I peptides are recognized by CD8+ T cells and the MHC class II by CD4+ T cells.
Antigen processing and presentation. A. Antigens originating in the cytoplasm are digested by the proteasome to peptides. The peptides are bound to the MHC class I molecules in the endoplasmic reticulum (ER) and transported to the surface for presentation. B. Antigens originating outside the cell are endocytosed and digested in the phagolysosome. The digested peptides are bound to MHC class II molecules in the ER and transported to the surface for presentation. MHC, major histocompatibility complex.
MHC I presents cytoplasmic peptides to CD8+
MHC II presents foreign peptides to CD4+
T cells originate in the bone marrow and migrate to the thymus for differentiation. Those that recognize self are destroyed. Those that survive are mature but still to be activated. T cells have specific TCRs on their surface, with binding sites extending to the outside (Figure 2–12). The two major types of T cells are helper T (CD4+) and cytotoxic (CD8+) T cells. The major roles of T cells in the immune response are as follows:
Recognition of peptide epitopes presented by MHC molecules on cell surfaces. This is followed by activation and clonal expansion of T cells in the case of epitopes associated with class II MHC molecules.
Production of cytokines that act as intercellular signals and mediate the activation and modulation of various aspects of the immune response and of nonspecific host defenses.
Direct killing of foreign cells, of host cells bearing foreign surface antigens along with class I MHC molecules (eg, some virally infected cells), and of some immunologically recognized tumor cells.
T-cell receptor and helper T activation. A. Structure of the T-cell antigen receptor. B. An antigen-presenting cell begins the activation process by displaying a peptide antigen fragment in its MHC class II molecule. A helper T cell is activated after the variable region of its receptor (Vα, Vβ) reacts with the fragment. (Reproduced with permission from Willey JM: Prescott, Harley, & Klein's Microbiology, 7th edition. McGraw-Hill, 2008.)
CD4+ Helper T Lymphocytes
Helper T cells (TH cells) are stimulated by antigen in the context of MHC class II presentation and are further marked by the presence of the CD4 cell surface antigen. If T cells are of the proper MHC background to recognize the antigen specifically, T-cell activation occurs. The antigen–MHC complex presented to a specific T cell by the macrophage is the specific signal that induces the T cell to become activated and divide. At this point, the helper T cells follow either the TH1 pathway toward cell-mediated immunity or the TH2 pathway toward antibody production and humoral immunity. Before this differentiation, the helper cells are sometimes referred to as TH0. The TH1 and TH2 responses are characterized by their own set of cytokines and biologic actions. In both pathways, this clonal expansion includes memory cells along with the T cells committed to effector functions. These pathways are illustrated in Figure 2–13.
T-cell responses. A virus is phagocytosed by a macrophage, and a small antigen fragment (peptide) is presented to naïve TH cells in association with class II MHC molecules. After activation, the TH0 cell may differentiate into a TH2 cell that secretes the cytokines that cause B-cell proliferation and subsequent secretion of specific antiviral antibodies. Alternately, they differentiate into TH1 cells that secrete cytokines, which regulate the proliferation of cytotoxic T cells (CTLs). Once a CTL proliferates and differentiates into an activated effector cell, it attacks and causes lysis or apoptosis of a virus-infected cell. (Reproduced with permission from Willey JM: Prescott, Harley, & Klein's Microbiology, 7th edition. McGraw-Hill, 2008.)
TH cells are stimulated by MHC II-presented antigen
TH1 to cell-mediated reactions
TH2 to antibody production
CD8+ Cytotoxic T Lymphocytes
CD8+ cytotoxic T lymphocytes (CTLs) are a second class of effector T cells. They are lethal to cells expressing the epitope against which they are directed when the epitope is presented by class I MHC molecules. They too have specific epitope recognition sites, but they are characterized by the CD8 cell surface marker; thus, they are referred to as CD8+ cytotoxic T cells. These cells recognize the association of antigenic epitopes with class I MHC molecules on a wide variety of cells of the body. In the case of virally infected cells, cytotoxic CD8+ cells prevent viral production and release by eliminating the host cell before viral synthesis or assembly is complete (Figure 2–14). The destruction of the virally infected cell is accomplished through a complement-like action mediated by perforins, which also facilitates entry into the cell of enzymes (granzymes) that activate apoptosis.
Cytotoxic T-cell (CTL) destruction virus-infected cells. A. Naïve CD8+ T cells are activated when they are exposed to antigen within a class I MHC molecule on an antigen-presenting cell. Antigen activation leads to development of effector CTL and memory cells. Effector CTLs and their memory cells subsequently react with antigen expressed in class I MHC molecules of any host cell to destroy it. T-cell cytotoxicity often involves the perforin pathway and leads to apoptosis or cytolysis. MHC, major histocompatibility complex. B. CTL (left) contacting target cell (right). C. Perforins form pores in target cell membrane. (Reproduced with permission from Willey JM: Prescott, Harley, & Klein's Microbiology, 7th edition. McGraw-Hill, 2008.)
CD8+ lymphocytes react with MHC I
Eliminate virally infected cells
A group of antigens have been termed superantigens because they stimulate a much larger number of T cells than would be predicted based on the specificity of combining site diversity. This causes a massive cytokine release. The action of superantigens is based on their ability to bind directly to MHC proteins and to particular Vβ regions of the TCR without involving the antigen-combining site. Individual superantigens recognize exposed portions defined by framework residues that are common to the structure of one or more Vβ regions. Any T cells bearing those Vβ sites may be directly stimulated. A variety of microbial products have been identified as superantigens. Superantigens are discussed further in Chapter 22 (see Figure 22–6) and in Chapters 24 and 25, describing their role in toxic shock syndromes caused by Staphylococcus aureus and group A streptococci.
Superantigens bind directly to MHC proteins and TCR Vβ region
Higher proportion of T cells are stimulated
In the control of infection, cell-mediated immunity is most important in the response to obligate or facultative intracellular pathogens. These include some slow-growing bacteria, such as the mycobacteria and fungi against which antibody responses appear to be ineffective. The mechanisms are complex and involve a number of cytokines with amplifying feedback mechanisms for their production. After the initial processing of antigen to stimulate activation of the antigen-recognizing CD4+ T cell, cytokine feedback from the CD4+ T cells to macrophages further increases their clonal expansion (including memory cells) and activates CD8+ (cytotoxic) T lymphocytes. Other cytokines from CD4+ T cells attract macrophages to the site of infection, hold them there, and activate them to greatly enhance microbiocidal activity. The sum of the individual and collaborative activities of T cells, macrophages, and their products is a progressive mobilization of a range of host defenses to the site of infection and greatly enhanced macrophage activity. In the case of tuberculosis, IFN-γ inhibits the replication of the mycobacteria inside macrophages. In viral infections, CD8+ cytotoxic lymphocytes destroy their cellular habitat leaving already assembled virions accessible to circulating antibody.
Of primary importance with intracellular pathogens
Helper and cytotoxic T lymphocytes interact
Macrophages are mobilized and enhanced
B CELLS AND ANTIBODY RESPONSES
B lymphocytes are the cells responsible for antibody responses. They develop from precursor cells in the bone marrow before migrating to other lymphoid tissues. Each mature cell of this series carries a specific epitope recognition site on its surface. This B-cell receptor is actually a monomer of one form of antibody (IgM) oriented with its binding sites facing outward. Upon binding antigen, the receptor-antigen complex is internalized for initiation of antibody production by the stimulated B cell. In this process, the B lymphocytes multiply, differentiating into either memory or plasma cells. Plasma cells are end cells adapted for secretion of large amounts of antibodies. In addition to their essential role in antibody production, B cells can present antigen to T cells.
B cells carry epitope recognition sites on their surface
Stimulated cells differentiate to form memory, plasma cells
There are two broad types of antigen triggering: T-dependent and T-independent. T-dependent reactions are those that are use collaboration between helper T cells and B cells to initiate the process of antibody production in the manner shown in Figure 2–13. This is the mechanism evoked by proteins and haptens bound to proteins. The response is strong and includes memory cells, therefore, it can be boosted in the case of immunization.
T-independent responses are those that do not require help by T cells to stimulate B-cell antibody production. It is evoked by large molecules with many repeating units such as polysaccharides. At first glance, this independence may seem to be an advantage, but T-independent responses are not the same as T-dependent responses. The antibody generally has a lower affinity for its antigen and a shorter duration in circulation. Memory cells are not produced, and T-independent responses mature more slowly than T-dependent responses. This delay in maturation may contribute to the increased susceptibility to some bacterial infections in early life. It certainly contributed to the failure of purified polysaccharide vaccines to effectively immunize children younger than 2 years. For use in children, these vaccines have been replaced with a hapten approach in which the polysaccharide is conjugated to protein. In this form, antibody generated by the T-dependent mechanism (protein carrier) still has specificity for the polysaccharide epitopes.
T-cell independent responses are weaker and lack memory
Poor response under 2 years of age
After challenge with foreign antigen, there is a lag period of 4 to 6 days before antibody can be detected in serum. This period reflects the events involved in the recognition of the antigen, its processing, and the specific activation of the cells of the immune system. The first event is the clearance of antigen from the circulation by what is essentially a metabolic process in which the antigen is recognized in a nonspecific sense and ingested. The vast preponderance of antigen ends up in circulating phagocytes or in stationary macrophages. The macrophages process the antigen; therefore, that immunogenic moieties can be presented to T cells, which then cause the B cells to produce immunoglobulins. The antibody-forming system is a learning system that responds to challenge by foreign molecules by producing large amounts of specific antibody. In addition, the affinity of its binding to the specifically recognized antigen often increases with time or secondary challenge.
Antigen processing causes delay in antibody response
Learning system increases affinity with time or secondary challenge
Antibodies belong to the immunoglobulin family of proteins, which appear in quantity in serum and on the surfaces of B cells. Of the five known structural types, three (IgG, IgM, and IgA) are involved in the defense against infection. The basic structure of an immunoglobulin is illustrated in Figure 2–15, which depicts an IgG molecule. Immunoglobulins have a basic tetrameric structure consisting of two light polypeptide chains and two heavy chains usually associated as light/heavy pairs by disulfide bonds. The two light/heavy pairs are covalently associated by disulfide bonds to form the tetramer. There are two types of light chains, κ and λ, which are the products of distinct genetic loci. The class or isotype of the immunoglobulin is defined by the type of heavy chain expressed.
Immunoglobulin G structure. The IgG molecule consists of two identical light chains and two identical heavy chains held together by disulfide bonds. (Reproduced with permission from Willey JM: Prescott, Harley, & Klein's Microbiology, 7th edition. McGraw-Hill, 2008.)
Immunoglobulins have tetrameric structure combining light chains and heavy chains
Isotypes are defined by type of heavy chain
The Y-shaped structure includes two antigen binding sites (Fab) formed by interaction of the variable domains of the heavy chain and the light chain. The stalk is called the Fc fragment. Antibodies carry out two broad sets of functions: the recognition function is the property of the Fab sites for antigen, and the effector functions are mediated by the constant regions of the heavy chains. Variations in the hypervariable region of the Fab-combining site due to mutations are called idiotypes. Antibodies combine with foreign antigens, but the actual destruction or removal of antigen requires the interaction of portions of the Fc fragment with other molecules such as complement components and phagocytes which have Fc receptors.
Fab sites bind antigen
Fc fragment recognized by complement, phagocytes
Combining site is idiotype
Figure 2–16 shows a schematic representation of a serum IgM immunoglobulin. This molecule consists of five subunits of the typical IgG molecule. The molecule occurs as a cyclic pentamer, and a J (joining) chain links the intact structure. When IgM is present on the surface of B cells where it serves as a primary receptor for antigen, it is present as a monomer. Other immunoglobulins showing a difference in arrangement from the typical IgG model are the IgA immunoglobulins. In serum, these immunoglobulins can occur as a monomer, but they can also occur in dimers in which the joining chain is required to stabilize the dimer. IgA molecules in the gut occur as dimers in which both the J chain and an additional polypeptide, termed the secretory component, are present in the complex.
Immunoglobulin M structure. The pentameric structure has disulfide bonds linking peptide chains shown in black; carbohydrate side chains are in red. The J chain links the molecule together. (Reproduced with permission from Willey JM: Prescott, Harley, & Klein's Microbiology, 7th edition. McGraw-Hill, 2008.)
Fab is antigen-binding region
IgM has five subunits
IgA is a monomer or dimer
Functional Properties of Immunoglobulins
Immunoglobulin G (IgG) is the most abundant immunoglobulin in health and provides the most extensive and long-lived antibody response to the various microbial and other antigens that are encountered throughout life. Although at least four subclasses of IgG have been characterized, they are grouped together for the purpose of this chapter. The IgG molecule is bivalent with two identical and specific combining sites. The Fc region does not vary with differences in specificity of combining sites of different antibody molecules. The Fc fragment binding sites for phagocytic cells are made available when the variable region of the antibody molecule has reacted with specific antigen, leaving the Fc facing outward.
Bivalent molecule with specific combining site and constant region
Constant region binds phagocytes
IgG antibody is characteristically formed in large amounts during the secondary response to an antigenic stimulus, and usually follows production of IgM (see Immunoglobulin M) in the course of a viral or bacterial infection. Memory cells are programmed for rapid IgG response when another antigenic stimulus of the same type occurs later. Immunoglobulin G antibodies are the most significant antibody class for neutralizing bacterial exotoxins and viruses often by blocking their attachment to cell receptors. Accelerated IgG responses from memory cell expansion frequently confer lifelong immunity when directed against microbial antigens that are determinants of virulence. IgG is the only immunoglobulin class able to cross the placental barrier and, thus, provides passive immune protection to the newborn in the form of maternal antibody.
Antibodies produced during secondary response neutralize toxins and viruses
Binding may block attachment receptor
Monomers of immunoglobulin M (IgM) constitute the specific epitope recognition sites on B cells that ultimately give rise to plasma cells producing one or another of the different immunoglobulin classes of antibody. Because of its many specific combining sites, IgM is particularly effective in agglutinating particles carrying epitopes against which it is directed. It also contains many sites for binding the first component of complement. These sites become available once the IgM molecule has reacted with antigen. IgM is particularly active in bringing about complement-mediated cytolytic damage to foreign antigen-bearing cells. It is less effective as an opsonizing antibody because its Fc portion is not available to phagocytes.
Effective agglutinating antibody
Binds complement at multiple sites
Immunoglobulin A (IgA) has a special role as a major determinant of so-called local immunity in protecting epithelial surfaces from colonization and infection. Certain B cells in lymphoid tissues adjacent to, or draining surface epithelia of the intestines, respiratory tract, and genitourinary tract, are encoded for specific IgA production. After antigenic stimulus, the clone expands locally, and some of the IgA-producing cells also migrate to other viscera and secretory glands. At the epithelia, two IgA molecules combine with another protein, termed the secretory piece, which is present on the surface of local epithelial cells. The complex, then termed secretory IgA (sIgA), passes through the cells into the mucous layer on the epithelial surface or into glandular secretions, where it exerts its protective effect. The secretory piece not only mediates secretion, but also protects the molecule against proteolysis by enzymes such as those present in the intestinal tract.
sIgA is produced at mucosal surfaces
Secretory piece combines molecules and resists proteolysis
The major role of sIgA is to prevent attachment of antigen-carrying particles to receptors on mucous membrane epithelia. Thus, in the case of bacteria and viruses, it reacts with surface antigens that mediate adhesion and colonization and prevents the establishment of local infection or invasion of the subepithelial tissues. sIgA can agglutinate particles, but has no Fc domain for activating the classic complement pathway; however, it can activate the alternative pathway. Reaction of IgA with antigen within the mucous membrane initiates an inflammatory reaction that helps mobilize other immunoglobulin and cellular defenses to the site of invasion. IgA response to an antigen is shorter lived than the IgG response.
Interferes with attachment of microbes to mucosal surfaces
The major events characterizing the time course of antibody production are illustrated in Figure 2–17 and summarized as follows: Initial contact with a new antigen evokes the primary response, which is characterized by a lag phase of approximately 1 week between the challenge and the detection of circulating antibodies. In general, the length of the lag phase depends on the immunogenicity of the stimulating antigen and the sensitivity of the detection system for the antibodies produced. Once antibody is detected in serum, the levels rise exponentially to attain a maximal steady state in approximately 3 weeks. These levels then decline gradually with time if no further antigenic stimulation is given. The first antibodies synthesized in the primary immune response are IgM and, then in the latter phase, IgG antibodies arise and eventually predominate. This transition is termed the IgM/IgG switch.
Antibody production and kinetics. The four phases of a primary antibody response correlate to the clonal expansion of the activated B cell, differentiation into plasma cells, and secretion of the antibody protein. The secondary response is much more rapid, and total antibody production is nearly 1000 times greater than that of the primary response. (Reproduced with permission from Willey JM: Prescott, Harley, & Klein's Microbiology, 7th edition. McGraw-Hill, 2008.)
After a lag phase, the primary response lasts for weeks and then declines
IgM response switches to IgG
After a subsequent exposure or booster injection of the same antigen, a different sequence called the secondary response or anamnestic response ensues. This response involves memory. In the secondary response, the lag time between the immunization and the appearance of antibody is shortened, the rate of exponential increase to the maximum steady-state level is more rapid, and the steady-state level itself is higher, representing a larger amount of antibody. Another key factor of the secondary response is that the antibodies formed are predominantly of the IgG class. In addition to higher levels, the secondary IgG antibodies have a higher affinity for their antigen. Figure 2–17 shows the participation of memory T cells created during the primary response in these reactions.
Secondary response is primarily IgG
Affinity for antigen is greater