Chapter 3 Immune System Biology II PDF

Chapter 3 Immune system Dr. Leong Chean Ring Biology II – Foundation of science Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Why an immune system? Attack from outside...

Chapter 3 Immune system Dr. Leong Chean Ring Biology II – Foundation of science Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Why an immune system? Attack from outside ❖ Lots of organisms what you for lunch! ❖ Animals must defend themselves against unwelcome invaders: viruses bacteria, prostists and fungi ❖ We are a tasty vitamin-packed meal No cell wall Traded mobility for susceptibility The immune system recognizes foreign bodies = “not self” and responds w the production of immune cells and proteins. Two major kinds of defense have evolved: innate immunity and acquired immunity. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Why an immune system? Attack from inside ❖ Also deal with abnormal body cells = may develop into cancers Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Are viruses alive? The characteristic of living things ❖ Are made up of cells. ❖ Reproduce ❖ Based on a universal genetic code ❖ Grow and develop ❖ Obtain and use materials and energy ❖ Respond to their environment ❖ Maintain stable internal environment ❖ Change over time https://www.youtube.com/watch?v=WHnpp823BW8 Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Diversity of viral shapes Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Virus life cycle Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings The Lytic Cycle Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings The Lysogenic Cycle Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Differences between virus and cells Vaccines Antibiotics Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Diseases caused by viruses Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Diseases caused by bacteria Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Diseases caused by protists Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Diseases caused by fungi Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings How do immune cells of animals recognize foreign cells? 1.5 µm Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings  Innate immunity is present before any exposure to pathogens and is effective from the time of birth. It involves nonspecific responses to pathogens. Innate immunity consists of external barriers plus internal cellular and chemical defenses. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings  Acquired immunity = adaptive immunity, develops after exposure to agents such as microbes, toxins, or other foreign substances. It involves a very specific response to pathogens. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Animal Immunity Pathogens (microorganisms and viruses) INNATE IMMUNITY Barrier defenses: Skin Recognition of traits Mucous membranes shared by broad ranges Secretions of pathogens, using a small set of receptors Non-specific Internal defenses: Phagocytic cells Rapid response Antimicrobial proteins Inflammatory response Natural killer cells ACQUIRED IMMUNITY Humoral response: Antibodies defend against Recognition of traits infection in body fluids. specific to particular pathogens, using a vast Cell-mediated response: array of receptors Cytotoxic lymphocytes defend against infection in body cells. Slower response Innate Immunity of Invertebrates In insects, an exoskeleton made of chitin forms the first barrier to pathogens. The digestive system is protected by low pH and lysozyme, an enzyme that digests microbial cell walls. Hemocytes circulate within hemolymph and carry out phagocytosis, the ingestion and digestion of foreign substances including bacteria. Hemocytes also secrete antimicrobial peptides that disrupt the plasma membranes of bacteria. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Barrier Defenses Barrier defenses include the skin and mucous membranes of the respiratory, urinary, and reproductive tracts. Mucus traps and allows for the removal of microbes. Many body fluids including saliva, mucus, and tears are hostile to microbes. The low pH of skin and the digestive system prevents growth of microbes. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings  2nd : Non-specific patrol (internal barrier) Cells & protein Cellular elements 45% Attack invaders that penetrate Cell type Number per µL (mm3) of blood Functions body’s outer barriers Erythrocytes 5–6 million Transport oxygen (red blood cells) and help transport ❖ Phagocytic cells carbon dioxide ❖ Anti-microbial protein Leukocytes 5,000–10,000 Defense and ❖ Inflammatoryy responses (white blood cells) immunity Lymphocyte Basophil Eosinophil Neutrophil Monocyte Platelets 250,000– Blood clotting 400,000 Phagocytic leukocytes (WBC) Phagocytes Neutrophils (“NEUTRalize” invader) ❖ Attracted by chemical signals released by damaged cells ❖ Enter infected tissue, engulf & ingest microbes ▪ Amoeba-like (fierce!) ▪ Lysosomes ▪ -3 day life span Macrophages ❖ “big eater” ❖ Bigger, long lived phagocytes Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Phagocytosis Microbes PHAGOCYTIC CELL Vacuole Lysosome Containing hydrolytic enzymes Cellular Innate Defenses White blood cells = leukocytes engulf pathogens in the body via phagocytosis. Groups of pathogens are recognized by TLR, Toll-like receptors. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings  EXTRACELLULAR TLR FLUID Lipopolysaccharide signaling Helper protein Flagellin TLR4 WHITE BLOOD CELL TLR5 VESICLE TLR9 CpG DNA TLR3 Inflammatory responses ds RNA Lymphatic Interstitial fluid Adenoid System Tonsil Blood Lymph capillary nodes Spleen Tissue Lymphatic cells vessel Peyer’s patches (small intestine) Appendix Lymphatic vessels Lymph Masses of node defensive cells Antimicrobial Peptides and Proteins Peptides and proteins function in innate defense by attacking microbes directly or impeding their reproduction. Interferon proteins provide innate defense against viruses and help activate macrophages. About 30 proteins make up the complement system, which causes lysis of invading cells and helps trigger inflammation. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Major events in a local Inflammatory Response 1. 2. 3. Pathogen Splinter Chemical Macrophage signals Fluid Mast cell Capillary Phagocytosis Red blood cells Phagocytic cell Damage to tissue triggers local inflammatory response ❖ Histamines & prostaglandins released ❖ Capillaries dilate , more permeable ❖ Lead to clot formation Increased blood supply ❖ Swelling, reedness & heat of inflammation & infection ❖ Delivers WBC, RBC, platelets, clotting factors. Inflammatory Responses Following an injury, mast cells release histamine, which promotes changes in blood vessels; this is part of the inflammatory response. These changes increase local blood supply and allow more phagocytes and antimicrobial proteins to enter tissues. Pus = a fluid rich in white blood cells, dead microbes, and cell debris, accumulates at the site of inflammation. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings  Inflammation can be either local or systemic (throughout the body). Fever is a systemic inflammatory response triggered by pyrogens released by macrophages, and toxins from pathogens. Septic shock is a life-threatening condition caused by an overwhelming inflammatory response. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings In Acquired Immunity, lymphocyte receptors provide pathogen-specific recognition White blood cells called lymphocytes recognize and respond to antigens, foreign molecules. Lymphocytes that mature in the thymus above the heart are called T cells, and those that mature in bone marrow are called B cells. Lymphocytes contribute to immunological memory, an enhanced response to a foreign molecule encountered previously. Cytokines are secreted by macrophages and dendritic cells to recruit and activate lymphocytes. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Acquired Immunity = Active Immunity: Specific B cells and T cells have receptor proteins that can bind to foreign molecules. Each individual lymphocyte is specialized to recognize a specific type of molecule. An antigen is any foreign molecule to which a lymphocyte responds. A single B cell or T cell has about 100,000 identical antigen receptors. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Antigen receptors on lymphocytes Antigen- Antigen- Antigen- binding binding site binding site site Disulfide bridge Variable regions V V C C Constant C C regions Light chain Transmembrane region Plasma membrane Heavy chains  chain  chain Disulfide bridge B cell Cytoplasm of B cell Cytoplasm of T cell T cell B cell receptor T cell receptor  All antigen receptors on a single lymphocyte recognize the same epitope, or antigenic determinant, on an antigen. B cells give rise to plasma cells, which secrete proteins called antibodies or immunoglobulins. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings  Epitopes = antigen determinants Antigen- binding Epitopes sites (antigenic determinants) Antigen-binding sites Antibody A Antigen Antibody C C C Antibody B The Antigen Receptors of B Cells and T Cells B cell receptors bind to specific, intact antigens. The B cell receptor consists of two identical heavy chains and two identical light chains. The tips of the chains form a constant (C) region, and each chain contains a variable (V) region, so named because its amino acid sequence varies extensively from one B cell to another. Secreted antibodies, or immunoglobulins, are structurally similar to B cell receptors but lack transmembrane regions that anchor receptors in the plasma membrane. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings  Each T cell receptor consists of two different polypeptide chains. The tips of the chain form a variable (V) region; the rest is a constant (C) region. T cells can bind to an antigen that is free or on the surface of a pathogen. T cells bind to antigen fragments presented on a host cell. These antigen fragments are bound to cell- surface proteins called MHC molecules. MHC molecules are so named because they are encoded by a family of genes (many unique / specific) called the Major Histocompatibility Complex. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings The Role of the MHC In infected cells, MHC molecules bind and transport antigen fragments to the cell surface, a process called antigen presentation. A nearby T cell can then detect the antigen fragment displayed on the cell’s surface. Depending on their source, peptide antigens are handled by different classes of MHC molecules. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Antigen Presentation by an MHC molecule Top view: binding surface exposed to antigen receptors Antigen Class I MHC Antigen molecule Plasma membrane of infected cell  Class I MHC molecules are found on almost all nucleated cells of the body. They display peptide antigens to cytotoxic T cells. Class II MHC molecules are found on specialized cells: macrophages, B cells, and activated T cells… Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings  Interaction of T cells with Antigen-Presenting Cells Infected cell Microbe Antigen- 1 Antigen presenting cell associates with MHC Antigen molecule fragment Antigen fragment 1 1 Class I MHC Class II MHC molecule 2 molecule 2 T cell T cell 2 T cell receptor receptor recognizes combination (a) Cytotoxic T cell (b) Helper T cell  Class II MHC molecules are located mainly on dendritic cells, macrophages, and B cells. Dendritic cells, macrophages, and B cells are antigen-presenting cells that display antigens on their surface to cytotoxic T cells and helper T cells. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Amplifying Lymphocytes by Clonal Selection In the body there are few lymphocytes with antigen receptors for any particular epitope. The binding of a mature lymphocyte to an antigen induces the lymphocyte to divide rapidly. This proliferation of lymphocytes is called clonal selection. Two types of clones are produced: short-lived activated effector cells (fight current battle) and long- lived memory cells… for future attacks by same pathogen. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Clonal Selection of B cells Antigen molecules B cells that differ in antigen specificity Antigen receptor Antibody molecules Clone of memory cells Clone of plasma cells = effectors Primary immune response Secondary immune response to to antigen A produces antigen A produces antibodies to A. antibodies to A. Primary immune response to antigen B produces antibodies to B. 104 Antibody concentration (arbitrary units) 103 Antibodies to A 102 Antibodies to B 101 100 0 7 14 21 28 35 42 49 56 Exposure Exposure to to antigen A antigens A and B Time (days)  The first exposure to a specific antigen represents the primary immune response. During this time, effector B cells = plasma cells are generated, and T cells are activated to their effector forms. In the secondary immune response = memory cells facilitate a faster, more efficient response. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Acquired immunity defends against infection of body cells and fluids Acquired immunity has two branches: the humoral immune response and the cell- mediated immune response. Humoral immune response involves activation and clonal selection of B cells, resulting in production of secreted antibodies. Cell-mediated immune response involves activation and clonal selection of cytotoxic T cells. Helper T cells aid both responses. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings  Humoral (antibody-mediated) immune response Cell-mediated immune response Key Antigen (1st exposure) + Stimulates Engulfed by Gives rise to Acquired Antigen- + + presenting cell Immune Response + B cell + Helper T cell + Cytotoxic T cell Memory Helper T cells + + + Antigen (2nd exposure) + Memory Active Plasma cells Memory B cells Cytotoxic T cells Cytotoxic T cells Secreted antibodies Defend against extracellular pathogens by binding to antigens, Defend against intracellular pathogens thereby neutralizing pathogens or making them better targets and cancer by binding to and lysing the for phagocytes and complement proteins. infected cells or cancer cells. Acquired Humoral (antibody-mediated) immune response Immune Key Antigen (1st exposure) Response + Stimulates Engulfed by Gives rise to Antigen- + presenting cell + B cell + Helper T cell Memory Helper T cells + + Antigen (2nd exposure) Plasma cells Memory + B cells Secreted antibodies Defend against extracellular pathogens Acquired Cell-mediated immune response Immune Antigen (1st exposure) Key Response + Stimulates Engulfed by Gives rise to Antigen- presenting cell + + Helper T cell + Cytotoxic T cell Memory Helper T cells + + Antigen (2nd exposure) Active + Cytotoxic T cells Memory Cytotoxic T cells Defend against intracellular pathogens  Helper T Cells: Respond to Nearly All Antigens A surface protein called CD4 binds the class II MHC molecule. This binding keeps the helper T cell joined to the antigen-presenting cell while activation occurs. Activated helper T cells secrete cytokines that stimulate other lymphocytes. Positive Feedback in the Immune System enhances the process until some endpoint or maximum rate is reached. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings  The central role of helper T cells in humoral and cell-mediated immune responses Antigen- presenting Peptide antigen cell Bacterium Class II MHC molecule CD4 TCR (T cell receptor) Helper T cell Cytokines + + Humoral Positive Feedback … Cell-mediated immunity + + immunity = secretion of B cell Cytotoxic T cell = attack on antibodies by infected cells. plasma cells. Cytotoxic T Cells: A Response to Infected Cells Cytotoxic T cells are the effector cells in cell- mediated immune response. Cytotoxic T cells make CD8, a surface protein that greatly enhances interaction between a target cell and a cytotoxic T cell. Binding to a class I MHC complex on an infected cell activates a cytotoxic T cell and makes it an active killer. The activated cytotoxic T cell secretes proteins that destroy the infected target cell. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings The killing action of cytotoxic T cells 3. lysis 1. 2. Released cytotoxic T cell Cytotoxic T cell Perforin Granzymes CD8 TCR Dying target cell Class I MHC Pore molecule Target Peptide cell antigen B Cells: A Response to Extracellular Pathogens The humoral response is characterized by secretion of antibodies by B cells. Activation of B cells is aided by cytokines and antigen binding to helper T cells. Clonal selection of B cells generates antibody- secreting plasma cells, the effector cells of humoral immunity. Positive Feedback … Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings B cell activation in the humoral immune response Antigen-presenting cell Bacterium Peptide B cell antigen Class II MHC molecule + Clone of plasma cells Secreted TCR CD4 Cytokines antibody molecules Endoplasmic reticulum of plasma cell Activated Helper T cell helper T cell Clone of memory B cells 2 µm Antibody Classes The five major classes of antibodies, or immunoglobulins, differ in distribution and function. Polyclonal antibodies are the products of many different clones of B cells following exposure to a microbial antigen. Monoclonal antibodies are prepared from a single clone of B cells grown in culture. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings The five Class of Immuno- globulin (Antibody) Distribution Function antibody, or IgM (pentamer) First Ig class produced after Promotes neutraliza- tion and cross- immunoglobulin initial exposure to antigen; then its concentration in the blood declines linking of antigens; very effective in complement system activation (Ig), classes J chain IgG Most abundant Ig Promotes opsoniza- (monomer) class in blood; tion, neutralization, also present in and cross-linking of tissue fluids antigens; less effec- tive in activation of complement system than IgM Only Ig class that crosses placenta, thus conferring passive immunity on fetus IgA Present in Provides localized (dimer) secretions such defense of mucous as tears, saliva, membranes by mucus, and cross-linking and J chain breast milk neutralization of antigens Presence in breast milk confers Secretory passive immunity component on nursing infant IgE Present in blood Triggers release from (monomer) at low concen- mast cells and trations basophils of hista- mine and other chemicals that cause allergic reactions IgD Present primarily Acts as antigen (monomer) on surface of receptor in the B cells that have antigen-stimulated not been exposed proliferation and to antigens differentiation of B cells (clonal Trans- selection) membrane region  Class of Immuno- Distribution Function globulin (Antibody) IgM First Ig class Promotes neutraliza- (pentamer) produced after tion and cross- initial exposure to linking of antigens; antigen; then its very effective in concentration in complement system the blood declines activation J chain  Class of Immuno- Distribution Function globulin (Antibody) IgG Most abundant Ig Promotes opsoniza- (monomer) class in blood; tion, neutralization, also present in and cross-linking of tissue fluids antigens; less effec- tive in activation of complement system than IgM Only Ig class that crosses placenta, thus conferring passive immunity on fetus  Class of Immuno- Distribution Function globulin (Antibody) IgA Present in Provides localized (dimer) secretions such defense of mucous as tears, saliva, membranes by mucus, and cross-linking and J chain breast milk neutralization of antigens Presence in breast milk confers Secretory passive immunity component on nursing infant  Class of Immuno- Distribution Function globulin (Antibody) IgE Present in blood Triggers release from (monomer) at low concen- mast cells and trations basophils of hista- mine and other chemicals that cause allergic reactions  Class of Immuno- Distribution Function globulin (Antibody) IgD Present primarily Acts as antigen (monomer) on surface of receptor in the B cells that have antigen-stimulated not been exposed proliferation and to antigens differentiation of B cells (clonal Trans- selection) membrane region Active Immunization Active immunity develops naturally in response to an infection. It can also develop following/ from immunization, also called vaccination. In immunization, a nonpathogenic form of a microbe or part of a microbe elicits an immune response to an immunological memory. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings  Passive Immunity Passive immunity provides immediate, short- term protection. It is conferred naturally when IgG crosses the placenta from mother to fetus or when IgA passes from mother to infant in breast milk. It can also be conferred artificially by injecting antibodies into a nonimmune person. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Passive immunization of an infant occurs during breast-feeding Immune Rejection Cells transferred from one person to another can be attacked by immune defenses. This complicates blood transfusions or the transplant of tissues or organs. MHC molecules are different among genetically nonidentical individuals. Differences in MHC molecules stimulate rejection of tissue grafts and organ transplants. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings  Chances of successful transplantation increase if donor and recipient MHC tissue types are well matched. Immunosuppressive drugs facilitate transplantation. Lymphocytes in bone marrow transplants may cause the donor tissue to reject the recipient. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Blood Groups Antigens on red blood cells surface determine whether a person has blood type A (A antigen), B (B antigen), AB (both A and B antigens), or O (neither antigen). Antibodies to nonself blood types exist in the body. Transfusion with incompatible blood leads to destruction of the transfused cells. Recipient-donor combinations can be fatal or safe. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings  Disruption in immune system function can elicit or exacerbate disease Some pathogens have evolved to diminish the effectiveness of host immune responses. If the delicate balance of the immune system is disrupted, effects range from minor to often fatal. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Allergies Allergies are exaggerated (hypersensitive) responses to antigens called allergens. In localized allergies such as hay fever, IgE antibodies produced after first exposure to an allergen attach to receptors on mast cells. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Mast cells, IgE, and the allergic response IgE Histamine Allergen Granule Mast cell  The next time the allergen enters the body, it binds to mast cell–associated IgE molecules. Mast cells release histamine and other mediators that cause vascular changes leading to typical allergy symptoms. An acute allergic response can lead to anaphylactic shock, a life-threatening reaction that can occur within seconds of allergen exposure. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Autoimmune Diseases In individuals with autoimmune diseases, the immune system loses tolerance for self and turns against certain molecules of the body. Autoimmune diseases include systemic lupus erythematosus, rheumatoid arthritis, insulin- dependent diabetes mellitus, and multiple sclerosis. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings X-ray of a hand deformed by rheumatoid arthritis Exertion, Stress, and the Immune System Moderate exercise improves immune system function. Psychological stress has been shown to disrupt hormonal, nervous, and immune systems. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Immunodeficiency Diseases Inborn immunodeficiency results from hereditary or developmental defects that prevent proper functioning of innate, humoral, and/or cell-mediated defenses. Acquired immunodeficiency results from exposure to chemical and biological agents. Acquired immunodeficiency syndrome (AIDS) is caused by a virus. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Acquired Immune System Evasion by Pathogens Pathogens have evolved mechanisms to attack immune responses. Through antigenic variation, some pathogens are able to change epitope expression and prevent recognition. The human influenza virus mutates rapidly, and new flu vaccines must be made each year. Human viruses occasionally exchange genes with the viruses of domesticated animals. This poses a danger as human immune systems are unable to recognize the new viral strain. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Latency Some viruses may remain in a host in an inactive state called latency. Herpes simplex viruses can be present in a human host without causing symptoms. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Attack on the Immune System: HIV Human immunodeficiency virus (HIV) infects helper T cells. The loss of helper T cells impairs both the humoral and cell-mediated immune responses and leads to AIDS. HIV eludes the immune system because of antigenic variation and an ability to remain latent while integrated into host DNA. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings The progress of an untreated HIV infection Latency AIDS Relative antibody concentration 800 Helper T cell concentration in blood (cells/mm3) Relative HIV 600 concentration Helper T cell concentration 400 200 0 0 1 2 3 4 5 6 7 8 9 10 Years after untreated infection