Chapter 21 Nonspecific Body Defenses and Immunity G.R. Pitts, J.R. Schiller, and James F. Thompson, Ph.D. Defense Systems 1. Innate (nonspecific) defense – External body membranes – Inflammation • Antimicrobial proteins, phagocytes and other cells 2. Adaptive (specific) defense – T cells and B cells Innate Defense System • Surface Barriers – First line of defense – Skin and mucosal membranes – Mechanical and chemical protection • Internal Nonspecific Defenses – Second line of defense 1. Phagocytes 2. Natural Killer cells (NK lymphocytes) 3. Inflammation 4. Antimicrobial proteins 5. Fever Innate Defense: Surface Barriers • Skin and mucosal membranes – Mechanical protection • Intact epidermis • Mucous membranes – line body cavities, mucus prevents drying, traps foreign things – nose hairs, respiratory tract cilia • Lacrimal apparatus -- tear glands and ducts – continually wash the eye to dilute microbial growth • Saliva - dilute microbes on the teeth, tongue, and gums • Urine - flow dilutes, and acid pH helps kill, microorganisms • Defecation and vomiting - expel toxic microbes Innate Defense: Surface Barriers – Chemical protection: helps prevents bacterial growth • Skin – sebum (unsaturated FA’s) forms oily layer – perspiration has fatty acids, salts (NaCl), and mildly acid pH • Lysozyme – in perspiration, tears, saliva, nasal secretions, other tissue fluids – enzyme breaks down bacterial cell walls • Hyaluronic acid – gel-like matrix in most connective tissues – slows the spread of many infectious agents • Gastric juice - stomach nearly sterile due to acid pH, ~2 • Vaginal secretions – mildly acid pH Innate Defense: Phagocytes • Macrophage are the chief phagocytic cell – Derived from monocytes • Free macrophages wander throughout a region in search of cellular debris • Kupffer cells (liver) and microglia (brain) are fixed macrophages • Neutrophils become phagocytic when encountering infectious material • Eosinophils are weakly phagocytic, deploy destructive granules against parasitic worms • Mast cells bind and ingest a wide range of bacteria Mechanism of Phagocytosis • Chemotaxis • Adherence – recognition of carbohydrate “signature” – Aided by opsonization • Ingestion • Digestion Innate Defense: Natural Killer Cells Natural Killer cells (NK lymphocytes) – Small, distinct group of large granular lymphocytes – Nonspecific killers which respond to the lack of self-antigens and to the presence of specific sugars before the antigen-specific immune system is activated – Able to kill virus-infected body cells and some tumor cells by releasing various defensive molecules (perforin) – not by phagocytosis – Secrete potent chemicals that enhance the inflammatory response Innate Defense: Inflammation 1. Inflammation • Signs: 1. Redness 2. Heat 3. Swelling 4. Pain 5. Loss of Function • Function: 1. Prevent spread of damage 2. Dispose of pathogens and debris 3. Set stage for tissue repair Inflammation Stage 1: Vasodilation and increased vessel permeability – Macrophages and cells lining the gastrointestinal and respiratory tracts bear TLRs that recognize specific classes of infecting microbes – TLR activation causes cytokine release • Promotes inflammation • chemoattractant – Mast cells secrete histamine – Other cells secrete various regulatory factors • Histamine, kinins, prostaglandins, leukotrienes, complement – Causes local vasodilation, resulting in http://www.komabiotech.co.kr/technical/review/toll_like_receptor.gif hyperemia – Increases capillary permeability, resulting in edema Inflammation: Stage 1 • Edema - A surge of protein-rich fluids into tissue spaces – Helps to dilute harmful substances – Brings in large quantities of oxygen and nutrients needed for repair – Allows entry of clotting proteins, which prevents the spread of bacteria Inflammation Stage 2. Phagocyte moblization 1. Leukocytosis-inducing factors: increase neutrophil production 2. Margination (pavementing) 3. Diapedesis (amoeboid movement) 4. Chemotaxis of WBCs • neutrophils – rapid arrival • monocytes – slower arrival Inflammation Stage 3. Tissue repair – Tissue regrowth and repair of damage or scar formation – Pus • dead phagocytes and other WBCs, damaged tissue, and perhaps microbes • if too numerous for effective removal by phagocytes, an abscess may develop Effects of Inflammation • Increased blood flow which results in increased local temperature and local cellular metabolism • Increased capillary permeability and phagocytic migration to the injured tissue Innate Defense: Antimicrobial Proteins • Enhance the innate defenses by: 1. Attacking microorganisms directly 2. Hindering microorganisms’ ability to reproduce • The most important antimicrobial proteins are: 1. Interferon 2. Complement proteins 3. Transferrins bind Fe2+ in plasma, inhibiting bacterial growth Interferon (IFN) • Produced by cells in response to viral infections • Diffuses to uninfected cells and binds to surface receptors – stimulates production of antiviral proteins like PKR. • PKR blocks viral replication – enhances macrophages and natural killer lymphocytes – inhibits growth of virally infected cells – suppresses growth of tumor cells • FDA-approved alpha IFN is used: – As an antiviral drug against hepatitis C virus – To treat genital warts caused by the herpes virus Innate Defense: Antimicrobial Proteins • Complement – 20 plasma and cell membrane proteins that exist in inactive forms – When activated, the complement system functions to “complement” or enhance certain immune, allergic, and inflammatory responses – Amplifies all aspects of the inflammatory response – Kills bacteria and certain other cell types (our cells are immune to complement) – Enhances the effectiveness of both nonspecific and specific defenses Complement Pathways Classical pathway is linked to the immune system – Requires binding of antibodies to antigens of invading organisms – Subsequently, C1 binds to the antigen-antibody complexes (complement fixation) Alternative pathway is triggered by interaction among factors B, D, and P, and polysaccharide molecules present on microorganisms (direct recognition of microbial antigens) Complement Pathways • Each pathway involves a cascade • Both pathways converge on C3, which cleaves into C3a and C3b • C3b initiates formation of a membrane attack complex (MAC) • MAC causes cell lysis by interfering with a cell’s ability to control intracellular [Ca2+] • C3b also causes opsonization • C3a causes inflammation Innate Defense: Fever • Pyrogens reset the temperature set-point in the hypothalamus • Inhibits some microbes from growing • Increases metabolic rate, which speeds up tissue repair • The liver and spleen to sequester iron and zinc (needed by microorganisms) • Increases effects of antimicrobial substances produced by the immune system • High fevers are dangerous Innate Defense System: Review • Surface Barriers – First line of defense – Skin and mucosal membranes – Mechanical and chemical protection • Internal Nonspecific Defenses – Second line of defense 1. Phagocytes 2. Natural Killer cells (NK lymphocytes) 3. Inflammation 4. Antimicrobial proteins 5. Fever Adaptive Defense • The adaptive immune system is a functional system that: – Acts to immobilize, neutralize, or destroy foreign substances – Amplifies the inflammatory response and activates complement – Is antigen-specific, systemic, and has memory • Recognizes specific foreign substances – Has two separate but overlapping arms • Humoral, or antibody-mediated immunity • Cellular, or cell-mediated immunity Adaptive Defense • Definitions: – Immunity: the ability of the body to defend itself against specific foreign invaders (molecules or cells) – Immunogenicity: the ability to stimulate proliferation of specific lymphocytes and antibody production – Reactivity: the ability to react with the products of the activated lymphocytes and the antibodies released in response to them Adaptive Defense • Definitions: – Specificity: the antigen triggers immune defenses (from lymphocytes) that respond only to the antigens of this foreign substance/cell – Memory: the immune system produces clones of specific memory lymphocytes (T & B) which react rapidly when the particular foreign substance/cell is encountered again – Specificity and memory differentiate this system from the non-specific system’s defenses Adaptive Defense • Antigen – any substance which provokes specific immune responses • Antigenic determinants – Parts of antigens that trigger the specific immune response – An antigen may be an Most “antigens” are entire microorganism or complex and express only small structures or multiple types of subregions of large antigenic determinants. molecules Adaptive Defense • Chemical nature of antigens – “Complete”: large, complex molecules - usually proteins (nucleo- , lipo-, glyco-) -- sometimes carbohydrates or lipids – Are immunogenic & reactive – “Incomplete”: smaller molecules (haptens) • react with antibodies but cannot cause an immune response without aid (protein carrier) • e.g., poison ivy allergen Adaptive Defense • Antigen receptor diversity – >1 billion different antigenic determinants are recognized by the body – Genetic recombination shuffles and reorganizes different Ab genes • Major histocompatibility complex antigens (MHC) – unique to each individual’s cells; help in identifying what is self versus foreign – 2 classes of MHC antigens • class I MHC – found on all body cells except RBC's • class II MHC - only on antigen presenting cells (APC’s), thymus cells, and activated T cells Cells of the Adaptive Immune System: Lymphocytes • Immature lymphocytes released from bone marrow are essentially identical • Whether a lymphocyte matures into a B cell or a T cell depends on where it becomes able to recognize a specific antigen (immunocompetent) – B cells mature in the bone marrow & oversee humoral immunity – T cells mature in the thymus & oversee cell-mediated immunity Immunocompetent B or T cells • Display a unique type of receptor for a specific antigen – Occurs before cells encounter antigens they may later attack – It is genes, not antigens, that determine which foreign substances our immune system will recognize and resist • Naive cells are exported to secondary lymphoid tissue where cells may encounter antigens – Mature into fully functional antigen-activated cells upon binding with their recognized antigen T Cells • T cells mature in the thymus under positive and negative selection pressures Survive – Positive selection – thymic cortex • Selects T cells with a weak response to self-antigens (MHC molecules) Apoptosis • These cells become both immunocompetent and self-tolerant • Non-selected cells die via apoptosis Apoptosis – Negative selection – inner portion of thymic cortex • Eliminates T cells that strongly react with self-antigens (other than the MHC) B Cells • B cells become immunocompetent and self- tolerant in bone marrow – Some self-reactive B cells are killed by apoptosis (clonal deletion) – Some self-reactive B cells modify the self-reactive antigen (receptor editing) • Some self-reactive B cells are released from the bone and are inactivated (anergy) Cells of the Adaptive Immune System: APCs • Antigen-presenting cells (APCs): – Engulf, process, and present parts of antigens – Do not respond to specific antigens – Play essential auxiliary roles in immunity – Dendritic (Langerhans) cells: connective tissue and epidermis – Macrophage – B lymphocytes The major initiators of adaptive immunity are DCs, which actively migrate to the lymph nodes and secondary lymphoid organs and present antigens to T and B cells Humoral Immune Response • Antigen challenge – first encounter between an antigen and a naive lymphocyte – Usually in the spleen or lymph node, but can occur in other lymphoid organs • If the lymphocyte is a B cell, a humoral immune response is provoked 1. Binding of the antigen to the receptor activates the lymphocyte 2. Clonal selection occurs 3. Antibodies are produced against the challenger Clonal Selection • Activated B cell grows & mitotically divides, forming clones bearing the same antigen- specific receptors Fate of the Clones • Most clone cells become plasma cells that secrete specific antibodies • Clones that do not become plasma cells become memory cells that can respond to subsequent exposures of the same antigen Immunological Memory • Primary immune response – cellular differentiation and proliferation, which occurs on the first exposure to a specific antigen – Lag period: 3 to 6 days after antigen challenge – Peak levels of plasma antibody are achieved in 10 days – Antibody levels then decline Immunological Memory • Secondary immune response – re-exposure to the same antigen – Sensitized memory cells respond within hours – Antibody levels peak in 2 to 3 days at much higher levels than in the primary response – Antibodies bind with greater affinity, and their levels in the blood can remain high for weeks to months Primary and Secondary Humoral Responses Soluble antibodies are the simplest ammunition of the immune response and interact in extracellular environments Types of Humoral Immunity • Active immunity: B cells encounter antigens and produce antibodies against them – Naturally acquired – response to a bacterial or viral infection – Artificially acquired – response to a vaccine of dead or attenuated (weakened) pathogens • Passive immunity: B cells are not challenged by antigens – Immunological memory does not occur – Protection ends when antigens naturally degrade in the body – Naturally acquired – from the mother to her fetus via the placenta or colostrum – Artificially acquired – from the injection of serum, such as gamma globulin Antibodies • Also called immunoglobulins – Constitute the gamma globulin portion of blood proteins – Are soluble proteins secreted by activated B cells and plasma cells in response to an antigen – Are capable of binding specifically with that antigen Basic Antibody Structure • Four polypeptide chains linked together with disulfide bonds • The four chains bound together form an antibody monomer • Each chain has a variable (V) region at one end and a constant (C) region at the other • Variable regions of the heavy and light chains combine to form the antigen-binding site Antibody Structure • Antibodies responding to different antigens have different V regions but the C region is the same for all antibodies in a given class • C regions form the stem of the Y-shaped antibody and: – Serve common functions in all antibodies – Dictate the cells and chemicals that the antibody can bind to – Determine the class of the antibody – Determine how the antibody class will function in elimination of antigens Classes of Antibodies • IgD: monomer attached to the surface of B cells, important in B cell activation • IgM: pentamer released by plasma cells during the primary immune response • IgG: monomer that is the most abundant and diverse antibody in primary and secondary response; crosses the placenta and confers passive immunity • IgA: dimer that helps prevent attachment of pathogens to epithelial cell surfaces • IgE: monomer that binds to mast cells and basophils, causing histamine release when activated Antibody Targets • Antibodies themselves do not destroy antigen; they inactivate and tag it for destruction • All antibodies form an antigen-antibody (immune) complex • Defensive mechanisms used by antibodies are neutralization, agglutination, precipitation, and complement fixation Antibody Mechanisms of Action 1. Neutralization: Antibodies bind to and block specific sites on viruses or exotoxins, thus preventing these antigens from binding to receptors on tissue cells 2. Agglutination: Antibodies bind the same determinant on more than one antigen – Makes antigen-antibody complexes that are cross-linked into large lattices – Cell-bound antigens are cross-linked, causing clumping (agglutination) 3. Precipitation – soluble molecules are cross-linked into large insoluble complexes Antibody Mechanisms of Action 4. Complement fixation is the main mechanism used against cellular antigens – Antibodies bound to cells change shape and expose complement binding sites – This triggers complement fixation and cell lysis – Complement activation: • Enhances the inflammatory response • Uses a positive feedback cycle to promote phagocytosis – Enlists more and more defensive elements Mechanisms of Antibody Action Figure 21.13 Monoclonal Antibodies • Commercially prepared antibodies are used: – To provide passive immunity – In research, clinical testing, and treatment of certain cancers • Monoclonal antibodies are pure antibody preparations – Specific for a single antigenic determinant – Produced from descendents of a single cell Cell-Mediated Immunity • Since antibodies are useless against intracellular antigens, cell-mediated immunity is needed • Two major populations of T cells mediate cellular immunity – CD4 cells (T4 cells) are primarily helper T cells (TH) – CD8 cells (T8 cells) are cytotoxic T cells (TC) that destroy cells harboring foreign antigens • Other types of T cells are: – Suppressor T cells (TS) – Memory T cells Importance of Cell-Mediated Immunity • T cells recognize and respond only to processed fragments of antigen displayed on the surface of body cells • T cells are best suited for cell-to-cell interactions, and target: – Cells infected with viruses, bacteria, or intracellular parasites – Abnormal or cancerous cells – Cells of infused or transplanted foreign tissue Cell -Mediated Immunity Basic steps 1. Recognition of antigen presented by an antigen- presenting cell by T cell receptors (TCR’s) 2. Proliferation and differentiation of T cells once activated 3. Production of a clone of identical effector T cells capable of recognizing initial activator (antigen) 4. Elimination of the foreign intruder Antigen Recognition and MHC Restriction • Allows the immune system to recognize the presence of intracellular microorganisms • Immunocompetent T cells are activated when the variable regions of their surface receptors bind to a recognized antigen and the T cells simultaneously recognize a self-antigen (an MHC protein) • MHC proteins are ignored by T cells if they are loaded with self protein fragments Antigen Recognition • If MHC proteins are complexed with endogenous or exogenous antigenic peptides, they: – Indicate the presence of intracellular infectious microorganisms – Act as antigen holders – Form the self part of the self-antiself complexes recognized by T cells • There are 2 classes of MHC proteins Class I MHC Proteins • Found on all cells, except RBCs • Always recognized by cytotoxic TC cells (CD8) • Display peptides from endogenous antigens – Endogenous antigens are: • Degraded by proteases and enter the endoplasmic reticulum • Transported via TAP (Transporter associated with Antigen Processing) • Loaded onto class I MHC molecules • Displayed on the cell surface in association with a class I MHC molecule Class I MHC Proteins Class II MHC Proteins 1. Found only on mature B cells, some T cells, and antigen-presenting cells 2. A phagosome containing pathogens (with exogenous antigens) merges with a lysosome 3. Class II MHC proteins are synthesized in the ER – Invariant protein prevents class II MHC proteins from binding to peptides in the endoplasmic reticulum 4. Class II MHC proteins migrate into the phagosomes where the antigen is degraded and the invariant chain is removed for peptide loading 5. Loaded class II MHC molecules then migrate to the cell membrane and display antigenic peptide for recognition by CD4 TH cells Class II MHC Proteins T Cell Activation- Step 1: Antigen Binding • T cell antigen receptors (TCRs): – Activated by binding to an antigen-MHC protein complex – Have variable and constant regions consisting of two chains (alpha and beta) – Linked to multiple intracellular signaling pathways • MHC restriction – Tc binds to antigens complexed with class I MHC – TH binds to antigens complexed with class II MHC • Mobile APCs (Langerhans/dendritic cells) quickly alert the body to the presence of antigen by migrating to the lymph nodes and presenting antigen – May display exogenous antigens from dying virus-infected cells or tumor cells on class I and II MHC proteins T Cell Activation- Step 1: Antigen Binding T Cell Activation- Step 2: Co-stimulation • T cells must bind to other surface receptors on an APC – Macrophage & dendritic cells produce surface B7 proteins when nonspecific defenses are mobilized – B7 binding with the CD28 receptor on the surface of T cells is a crucial co-stimulatory signal • After co-stimulation, other cytokines such as interleukin 1 and 2 stimulate proliferation and differentiation of T cells T Cell Activation- Step 2: Co-stimulation • Without co-stimulation, T cells: – Become tolerant to that antigen (anergy) – Are unable to divide – Do not secrete cytokines • T cells that are activated: – Enlarge, proliferate, and form clones – Differentiate and perform functions according to their T cell class T Cell Activation- Step 2: Co-stimulation • Primary T cell response peaks within a week after signal exposure • T cells then undergo apoptosis between days 7 and 30 • Effector activity wanes as the amount of antigen declines • The disposal of activated effector cells is a protective mechanism for the body • Memory T cells remain and mediate secondary responses to the same antigen Helper T Cells (TH) • Regulatory cells that play a central role in the immune response • Once primed by APC presentation of antigen, they: – Chemically or directly stimulate proliferation of other T cells – Stimulate B cells that have already become bound to antigen • There is NO immune response without TH Helper T Cell • TH cells interact directly with B cells that have antigen fragments on their surfaces bound to MHC II receptors • TH cells stimulate B cells to divide more rapidly and begin antibody formation • B cells may be activated without TH cells by binding to T cell–independent antigens • Most antigens, however, require TH co-stimulation to activate B cells • Cytokines released by TH amplify nonspecific defenses Cytotoxic T Cell (Tc) • TC cells, or killer T cells, are the only T cells that can directly attack and kill other cells • They circulate throughout the body in search of body cells that display the antigen to which they have been sensitized • Their targets include: – Virus-infected cells – Cells with intracellular bacteria or parasites – Cancer cells – Foreign cells from blood transfusions or transplants Cytotoxic T Cells • Bind to self-antiself complexes on all body cells • Infected or abnormal cells can be destroyed as long as appropriate antigen and co-stimulatory stimuli (e.g., IL-2) are present • In contrast, natural killer cells activate their killing machinery when they bind to MICA receptor – MICA receptor – MHC-related cell surface protein in cancer cells, virus-infected cells, and cells of transplanted organs Mechanisms of Tc Action • In some cases, TC cells: – Bind to the target cell and release perforin into its membrane • In the presence of Ca2+ perforin causes cell lysis by creating transmembrane pores Other Tc and Ts Cells • Other TC cells induce cell death by: – Secreting lymphotoxin, which fragments the target cell’s DNA – Secreting gamma interferon, which stimulates phagocytosis by macrophages • Suppressor T cells (TS) – regulatory cells that release cytokines, which suppress the activity of both T cells and B cells Summary of the Primary Immune Response Adaptive Immunity: Summary • Two-fisted defensive system that uses lymphocytes, APCs, and specific molecules to identify and destroy nonself particles • Its response depends upon the ability of its cells to: – Recognize foreign substances (antigens) by binding to them – Communicate with one another so that the whole system mounts a response specific to those antigens Adaptive Immunity: Summary To start an immune response, B and T cells must recognize foreign antigen B cells can recognize and bind to certain antigens in the blood or the extracellular fluid (ECF) More often, B and T cells only recognize antigen (protein fragments) when Ag is presented by the phagocytes in combination with MHC Class II surface markers Organ Transplants • The four major types of grafts are: – Autografts – graft transplanted from one site on the body to another in the same person – Isografts – grafts between identical twins – Allografts – transplants between individuals that are not identical twins, but belong to same species – Xenografts – grafts taken from another animal species Prevention of Rejection • Prevention of tissue rejection is accomplished by using immunosuppressive drugs • However, these drugs depress patient’s immune system so it cannot fight off foreign agents Pathologies: Immunodeficiencies • Human Immunodeficiency Virus – HIV enters certain cell types by receptor mediated endocytosis • infects primarily helper T cells • attaches to the CD4 protein on cell surface – A retrovirus • carries its genetic material as RNA • inserts its genetic material into host cell DNA with the enzyme reverse transcriptase • cell makes copies of the virus, releases them for further infection – May be carried silently in cells for years, being passed on during ordinary mitosis – Activation of HIV life cycle destroys THelper cells – Weakened immune response to all foreign invaders, benign or aggressive Pathologies: Autoimmune Diseases • Multiple Sclerosis (MS) – myelin sheath (white matter) attacked and destroyed • Myasthenia Gravis – neuro-muscular junction of skeletal muscle attacked and destroyed • Graves Disease – thyroid cells’ TSH receptor attacked and stimulated causing excess thyroid hormone (T3 & T4) production • Type I Diabetes - destruction of pancreatic islet cells Pathologies: Autoimmune Diseases • Systemic Lupus Erythematosus (SLE) – generalized attack on connective tissues and nuclear antigens • Glomerulonephritis - destruction of the glomerular capillaries causes impaired renal function • Rheumatoid Arthritis - destruction of the synovial membranes in joints Pathologies: Cancer • The immune system probably evolved first to respond to cancer cells – when a new cancer cell develops, new surface marker proteins (tumor antigens) often appear – if the immune system recognizes these new surface markers as non-self, it will destroy the cell expressing them – this immune surveillance is most effective in eliminating virus-induced tumor cells because they tend to express viral antigens which are not “self” • Leukemias and Lymphomas – cancers of leukocytes Pathologies: Hypersensitivities • Immediate hypersensitivities (allergies) – First exposure merely sensitizes one to an allergen • APCs digest and present allergen • Abnormally large numbers of TH cells secrete IL-4, stimulating B cells to secrete IgE • IgE attaches to mast cells and basophils – Later exposures produce dramatic responses • Antigen binds to IgE on mast cells and basophils • Cells release huge amounts of histamine and other inflammatory chemicals • Local reactions - surface exposure • Systemic Reactions – body-wide – Anaphylactic shock – penicillin, bee stings, etc. Pathologies: Subacute Hypersensitivities • Caused by IgG and IgM • Occurs 1-3 hr after exposure and lasts 10-15 hr • Cytotoxic reactions – Ab bind to Ag on specific cells causing phagocytosis and complement-activated lysis – May occur after transfusion of mismatched blood • Immune-complex hypersensitivities – Ag’s are widely distributed or insoluble Ag-Ab complexes can’t be removed – Intense inflammation – Severe damage to local tissue – Also involved in autoimmune diseases Pathologies: Delayed Hypersensitivities • Occurs 1-3 days after exposure • Cell-mediated immune response • May be transferred with blood transfusions • Causes mild swelling to serious cytotoxic tissue damage (poison ivy, latex gloves, etc.) End Chapter 21
"Nonspecific Body Defenses and Immunity"