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Chapter 22 Defenses against Disease Lectures by Gregory Ahearn University of North Florida Copyright © 2009 Pearson Education, Inc.. 22.1 How Does The Body Defend Against Invasion? There are three lines of defense that the body has against invading organisms that could kill us. • External barriers, such as the skin, keep microbes out of the body. • Nonspecific internal defenses combat invading microbes. • The immune system targets specific microbes. Copyright © 2009 Pearson Education Inc. 22.1 How Does The Body Defend Against Invasion? Levels of defense against infection Nonspecific External Barriers Skin Mucous membranes If these barriers are penetrated, the body responds with Nonspecific Internal Defenses Phagocytic and natural killer cells Inflammation Fever If the nonspecific defenses are insufficient, the body responds with Specific Immune Response Cell-mediated immunity Humoral immunity Fig. 22-1 Copyright © 2009 Pearson Education Inc. 22.1 How Does The Body Defend Against Invasion? The skin and mucous membranes are nonspecific external barriers to invasion. • The skin is dry and does not provide the water or nutrients that microbes need to survive. • Skin also secretes sweat and oil, which contain natural antibiotics, such as lactic acid, that kills bacteria and fungus. Copyright © 2009 Pearson Education Inc. 22.1 How Does The Body Defend Against Invasion? The skin and mucous membranes are nonspecific external barriers to invasion (continued). • The stomach is acidic and has proteindigesting enzymes. • Mucous membranes of the gut, respiratory tract, and urogenital tracts secrete mucus with antibacterial enzymes, such as lysozyme, that destroys bacteria. Copyright © 2009 Pearson Education Inc. 22.1 How Does The Body Defend Against Invasion? The protective function of mucus Bacteria trapped by mucus and cilia Fig. 22-2 Copyright © 2009 Pearson Education Inc. 22.1 How Does The Body Defend Against Invasion? Nonspecific internal defenses combat invaders. • These defenses attack a wide variety of microbes rather than target specific invaders; they fall into three main categories: • Phagocytic cells • Natural killer cells • The inflammatory response Copyright © 2009 Pearson Education Inc. 22.1 How Does The Body Defend Against Invasion? Phagocytic cells and natural killer cells destroy invading microbes. • Phagocytic cells engulf and digest microbes; one important type is the macrophage. • Macrophages ingest microbes by phagocytosis, consuming bacteria and foreign substances that penetrate the mucous membranes of the skin. Copyright © 2009 Pearson Education Inc. 22.1 How Does The Body Defend Against Invasion? The attack of the macrophages Bacteria visible through a hole in the macrophage membrane (a) A macrophage leaves a capillary and enters a wound (b) A macrophage stuffed with bacteria that it has ingested Fig. 22-3 Copyright © 2009 Pearson Education Inc. 22.1 How Does The Body Defend Against Invasion? Phagocytic cells and natural killer cells destroy invading microbes (continued). • Viruses leave their proteins on the surface of infected body cells and are recognized by natural killer cells. • Natural killer cells destroy infected body cells by secreting pore-forming proteins that attach to the infected cells plasma membrane and make holes through which the cytoplasm leaks, killing the cell and the viruses it contains. Copyright © 2009 Pearson Education Inc. 22.1 How Does The Body Defend Against Invasion? The inflammatory response defends against local infections. • Damaged cells release histamine into the wound. • Histamine increases blood flow to the area of the wound. • The area around the wound becomes red and swollen. Copyright © 2009 Pearson Education Inc. 22.1 How Does The Body Defend Against Invasion? The inflammatory response defends against local infections (continued). • Other chemicals from the wound cause blood clotting, which closes off the wound to additional microbes. • Still other chemicals from the wound attract macrophages, that eat the bacteria. Copyright © 2009 Pearson Education Inc. 22.1 How Does The Body Defend Against Invasion? The inflammatory response Tissue damage carries bacteria into the wound Wounded cells release chemicals (red) that stimulate mast cells Mast cells release histamine (blue) Histamine increases capillary blood flow and permeability Phagocytes leave the capillaries and ingest bacteria and dead cells Fig. 22-4 Copyright © 2009 Pearson Education Inc. 22.1 How Does The Body Defend Against Invasion? PLAY Animation—Inflammation Copyright © 2009 Pearson Education Inc. 22.1 How Does The Body Defend Against Invasion? Fever combats large-scale infections. • A fever is produced after a population of microbes becomes established; it slows microbial reproduction and increases the body’s defenses. • Macrophages release hormones, called endogenous pyrogens, as they respond to an infection. Copyright © 2009 Pearson Education Inc. 22.1 How Does The Body Defend Against Invasion? Fever combats large-scale infections (continued). • These endogenous pyrogens travel to hypothalamus (the body’s thermostat) and alter body temperature, causing a fever; fever slows bacterial reproduction. • Fever helps fight viral infections by inducing infected cells to release interferon, which helps uninfected cells resist viral attack. Copyright © 2009 Pearson Education Inc. 22.2 What Are The Key Characteristics Of The Immune System? The immune system targets specific invading organisms. • It is composed of an army of separate cells that is coordinated in its actions through the use of hormones, receptors, cells, antigens, and antibodies. Copyright © 2009 Pearson Education Inc. 22.2 What Are The Key Characteristics Of The Immune System? The immune system targets specific invading organisms (continued). • It attacks one kind of microbe, overcomes it, and provides future protection against that particular microbe but to no others. • That is why the immune response is specific in its effect. Copyright © 2009 Pearson Education Inc. 22.2 What Are The Key Characteristics Of The Immune System? The immune system consists of cells and molecules dispersed throughout the body. • The immune system includes blood cells called lymphocytes that are clustered in the thymus, lymph nodes, spleen, and throughout the blood. • The immune cells produce proteins called antibodies and cytokines, which help destroy microbes. Copyright © 2009 Pearson Education Inc. 22.2 What Are The Key Characteristics Of The Immune System? Copyright © 2009 Pearson Education Inc. 22.2 What Are The Key Characteristics Of The Immune System? Immune cells originate in the bone marrow. • Two important lymphocytes are the B cells and T cells, which originate in the bone marrow and bring about the immune response. • Step 1: recognition of the invader • Step 2: launch the attack • Step 3: retention of a memory of the invader for future protection Copyright © 2009 Pearson Education Inc. 22.3 How Does The Immune System Recognize Invaders? Immune cells recognize invader’s complex molecules. • Invading microbes have surface proteins that act as antigens against which the immune cells generate other specific proteins, called antibodies. Copyright © 2009 Pearson Education Inc. 22.3 How Does The Immune System Recognize Invaders? Antibodies and T-cell receptors recognize and bind to antigens. • Antibody proteins are produced by B cells, and may remain attached to the surface of the cell that produced them or may be released into the blood plasma. • T-cell receptor proteins are produced by T cells and always remain attached to the surfaces of the T cells that produced them; they are never secreted into the plasma. Copyright © 2009 Pearson Education Inc. 22.3 How Does The Immune System Recognize Invaders? Antibodies both recognize and help to destroy invaders. • Antibodies are Y-shaped molecules composed of a pair of large (heavy) chains and a pair of small (light) chains. • Both heavy and light chains have constant and variable regions each. Copyright © 2009 Pearson Education Inc. 22.3 How Does The Immune System Recognize Invaders? Antibody structure antigen light chain heavy chain ss Variable regions form antigen binding sites Constant regions are the same in all antibodies of a given type Fig. 22-5 Copyright © 2009 Pearson Education Inc. 22.3 How Does The Immune System Recognize Invaders? Antibodies both recognize and help to destroy invaders (continued). • The variable regions form binding sites for specific antigens on microbes. • After the B cell antibody binds to an antigen on a microbe, the B cell engulfs the antigenbearing microbe and removes it. Copyright © 2009 Pearson Education Inc. 22.3 How Does The Immune System Recognize Invaders? Antibodies B cell antibody antigen (a) Antibody receptor function macrophage antibody antigen (b) Antibody effector function Fig. 22-6 Copyright © 2009 Pearson Education Inc. 22.3 How Does The Immune System Recognize Invaders? T-cell receptors recognize invaders and help to trigger the immune response. • T-cell receptors are only found on the surfaces of T cells, and have both similarities and differences from antibodies. • Like antibodies, they consist of peptide chains that form highly specific binding sites for antigens. Copyright © 2009 Pearson Education Inc. 22.3 How Does The Immune System Recognize Invaders? T-cell receptors recognize invaders and help to trigger the immune response (continued). • Unlike antibodies, T-cell receptors are not released into the bloodstream. • T-cell receptors trigger a response in the T cell only when it encounters an antigen on the surface of a cancerous or infected cell. Copyright © 2009 Pearson Education Inc. 22.3 How Does The Immune System Recognize Invaders? The immune system can recognize millions of different molecules. • Immune cells produce millions of types of antibodies and T-cell receptors capable of binding to almost all of the possible antigens a person might encounter. Copyright © 2009 Pearson Education Inc. 22.3 How Does The Immune System Recognize Invaders? The immune system can recognize millions of different molecules (continued). • Antibodies and T-cell receptors are proteins, and proteins are encoded by genes; there are 21,000 genes in the human genome. • This means that a relatively small number of genes must code for millions of antibodies and T-cell receptors. Copyright © 2009 Pearson Education Inc. 22.3 How Does The Immune System Recognize Invaders? Antibody genes are assembled from several segments of DNA. • B cell genes do not code for entire antibodies, but instead, code for antibody fragments that can be pieced together in millions of combinations. • Each B cell has DNA to produce the constant regions of antibodies but different pieces of DNA for the variable parts, so that each B cell makes a unique antibody. Copyright © 2009 Pearson Education Inc. 22.3 How Does The Immune System Recognize Invaders? Recombination of segments of DNA from antibody genes heavy chain V2 V1 V3 V4 light chain V200 V2 V1 V3 D1 D2 V4 D50 J1 J1 V150 J2 J2 J3 J4 J6 CM CD CG CE CA J5 CK (a) Genes for parts of the heavy chain (top) and light chain (bottom) of antibodies heavy chain V2 D11 J4 light chain V80 J2 CG V87 D8 CK J1 V101 J5 CG V111 D40 J1 CK Cell 1 Cell 2 (b) Complete antibody genes in three different B cells V2 D11 J4 V80 J2 CK V80 J2 CK CG CG Cell 1 V87 D8 J1 V101 J5 CK V101 J5 CG Cell 2 CK Cell 3 CK CG J1 V6 CG V111 D40 J1 V6 J1 CK V6 J1 CK CG CG Cell 3 (c) Antibodies synthesized by these three B cells Fig. 22-7 Copyright © 2009 Pearson Education Inc. 22.3 How Does The Immune System Recognize Invaders? Antibody genes are assembled from several segments of DNA (continued). • Some immunologists think that perhaps 15–20 billion different antibodies are possible. • T-cell receptors are made of different genes, but the process is similar. • There are more parts available for T-cell receptor genes, so there may be as many as a quadrillion (1015) different possible T-cell receptors! Copyright © 2009 Pearson Education Inc. 22.3 How Does The Immune System Recognize Invaders? Antibodies are not tailor-made for antigens. • The immune system does not design antibodies to fit invading antigens. • Instead, it randomly synthesizes millions of different antibodies, and this array is simply there, waiting for an appropriate antigen. • Virtually every antigen can be bound by at least a few antibodies because of the immense numbers of antibodies present in the body. Copyright © 2009 Pearson Education Inc. 22.3 How Does The Immune System Recognize Invaders? PLAY Animation—The Immune Response Copyright © 2009 Pearson Education Inc. 22.3 How Does The Immune System Recognize Invaders? The immune system distinguishes “self” from “non-self”. • It does so by destroying immune cells that respond to the body’s own molecules and retaining those that do not. • Some cell-surface proteins, called the major histocompatibility complex (MHC), are unique to each person. • Transplants are rejected because the immune system of the recipient recognizes the MHC of the donor as foreign antigens. Copyright © 2009 Pearson Education Inc. 22.4 How Does The Immune System Launch An Attack? The immune system launches two types of attack on invading microbes. • Humoral immunity: B cells and the antibodies they secrete attack invaders before they enter body cells. • Cell-mediated immunity: T-cells attack invaders after they have entered body cells. Copyright © 2009 Pearson Education Inc. 22.4 How Does The Immune System Launch An Attack? An immune response takes time to develop. • Although the immune system has millions of different types of antibodies and T-cell receptors, there are only a few of each type at any one time in the body. • Therefore, the immune system takes time to recognize the invader, multiply, and differentiate. Copyright © 2009 Pearson Education Inc. 22.4 How Does The Immune System Launch An Attack? It usually takes one or two weeks to mount a good response; it is then a race to overcome the multiplying microbes. immune response (amount of antibody produced) first exposure to antigen 0 1 2 3 4 5 6 7 8 time since exposure (weeks) 9 Fig. 22-8 Copyright © 2009 Pearson Education Inc. 22.4 How Does The Immune System Launch An Attack? Humoral immunity is produced by antibodies dissolved in blood. • The B cell that binds to a particular antigen starts to divide very rapidly, making more of itself with its particular antibody; this is called clonal selection. • The daughter cells differentiate into two kinds of cells: memory B cells and plasma cells. Copyright © 2009 Pearson Education Inc. 22.4 How Does The Immune System Launch An Attack? Clonal selection among B cells by invading antigens B cell plasma cell endoplasmic reticulum (b) Selected B cells differentiate into plasma cells Fig. 22-9 Copyright © 2009 Pearson Education Inc. 22.4 How Does The Immune System Launch An Attack? Humoral immunity is produced by antibodies dissolved in blood (continued). • Plasma cells make thousands of copies of the antibody that originally bound the antigen. • Memory B cells do not release antibodies, but rather, remember the antigen that started the process for a future immunity event. Copyright © 2009 Pearson Education Inc. 22.4 How Does The Immune System Launch An Attack? Humoral antibodies have multiple modes of action. • Humoral immunity is produced by antibodies in the blood, which combats invading molecules or microbes in three ways. Copyright © 2009 Pearson Education Inc. 22.4 How Does The Immune System Launch An Attack? Circulating antibodies may bind to a foreign snake venom molecule, virus, enzyme or cell and render it harmless, a process called neutralization. active site antibody Antibodies block the active site of the toxic enzymes in snake venom Fig. 22-10 Copyright © 2009 Pearson Education Inc. 22.4 How Does The Immune System Launch An Attack? antibodies Antibodies may coat the Variable regions of surface of an antibodies bind to a microbe microbe; the constant regions are exposed invading molecules, virus, or cell, Constant regions of make it easier the antibodies bind to the microbe surface of a macrophage, promoting phagocytosis for phagocytic cells to destroy them. macrophage Fig. 22-11 Copyright © 2009 Pearson Education Inc. 22.4 How Does The Immune System Launch An Attack? When antibodies bind to antigens on the surface of a microbe, they attract other proteins, called complement proteins. • Some complement proteins punch holes in the plasma membrane of the microbe. • Others promote phagocytosis of the invaders. Copyright © 2009 Pearson Education Inc. 22.4 How Does The Immune System Launch An Attack? Humoral immunity fights invaders that are outside cells. • Because antibodies are large proteins that usually do not cross plasma membranes, humoral immunity defends against invaders that are in the blood or extracellular fluid. • Bacteria, toxic molecules that are in venoms or are released by bacteria, some fungi, and protists are all susceptible to the humoral immune response. • Viruses that penetrate the body’s cells are only vulnerable to antibodies when they are outside a body cell, and are safe when they are inside a cell. Copyright © 2009 Pearson Education Inc. 22.4 How Does The Immune System Launch An Attack? Cell-mediated immunity is produced by cytotoxic T cells. • Cytotoxic T cells are the body’s defense against cancerous cells or cells invaded by viruses. • T-cell receptors on T-cell plasma membranes bind to viral antigens on the surface of an infected cell. • After binding to a viral antigen, the T cell releases a pore-forming protein that punches holes in the infected cell’s plasma membrane, killing the infected cells. • This prevents the replication of the enclosed viruses. Copyright © 2009 Pearson Education Inc. 22.4 How Does The Immune System Launch An Attack? Both humoral and cell-mediated immunity are enhanced by helper T cells. • Helper T cells bear receptors that bind to antigens, either on the surfaces of infected cells or of macrophages that have engulfed and digested invading microbes. • When a helper T cell receptor binds to an appropriate invading cell antigen, the T cells multiply rapidly and release chemicals, called cytokines. • Cytokines stimulate the division of B cells and cytotoxic T cells, which respond to the same invasion. Copyright © 2009 Pearson Education Inc. 22.4 How Does The Immune System Launch An Attack? Both humoral and cell-mediated immunity are enhanced by helper T cells (continued). • Both B cells and cytotoxic T cells only make a significant defense against disease if they receive stimulation by cytokines from helper T cells. • Human immunodeficiency virus (HIV), which causes AIDS, kills off helper T cells, leading to an ineffective defense against many diseases. Copyright © 2009 Pearson Education Inc. HUMORAL IMMUNITY Targets invaders outside cells (e.g., viruses, bacteria, fungi, protists, and toxins) HELPER T CELLS CELL-MEDIATED IMMUNITY Stimulate both humoral and cell-mediated immunity by releasing cytokines Targets defective body cells (e.g., infected cells and cancer cells), transplants virus viral antigen B-cell antibodies bind to viral antigens and stimulate the B cells to divide and differentiate macrophage Viral antigens presented on the surfaces of macrophages and infected cells infected cell T-cell receptors bind to viral antigens antibody B cell helper T cell cytotoxic T cell Cytokines released by helper T cells stimulate B cells and cytotoxic T cells plasma cell memory B cell memory helper T cell memory cytotoxic T cell cytotoxic T cell infected cell Plasma cells secrete antibodies into the blood and extracellular fluid Memory cells confer future immunity to this virus Cytotoxic T cells release poreforming proteins that destroy infected cells Fig. 22-12 Copyright © 2009 Pearson Education Inc. 22.5 How Does The Immune System Remember Its Past Victories? During the early stages of a disease—while B cells, cytotoxic T cell, and helper T cells are dividing rapidly—some of their daughter cells differentiate into memory cells that may survive for years. • If the body is re-invaded by this same type of microbe, the memory cells will recognize the invader and mount an immune response. Copyright © 2009 Pearson Education Inc. 22.5 How Does The Immune System Remember Its Past Victories? At a second infection, memory cells produce a second immune response that is so fast and large that the microbes are destroyed before any noticeable symptoms appear. immune response (amount of antibody produced) interval: months or years first exposure 0 Copyright © 2009 Pearson Education Inc. 1 2 3 second exposure 0 1 time since exposure (weeks) 2 3 Fig. 22-13 22.5 How Does The Immune System Remember Its Past Victories? PLAY Animation—Humoral vs. Cell-mediated Immunity Copyright © 2009 Pearson Education Inc. 22.5 How Does The Immune System Remember Its Past Victories? PLAY Animation—Humoral Immunity Copyright © 2009 Pearson Education Inc. 22.6 How Does Medical Care Assist The Immune Response? Antibiotics slow down microbial reproduction. • Organisms affected by antibiotics include bacteria, fungi, and protists. • Antibiotics do not destroy every single disease-causing microbe in the body, but they may kill enough of them to give the immune system time to finish the job. • Some microbes become resistant to antibiotics. Copyright © 2009 Pearson Education Inc. 22.6 How Does Medical Care Assist The Immune Response? Drugs are available that target different stages of a viral cycle of infection, which include the following: • Attachment to a host cell • Replication of viral parts using the host cell’s machinery • Assembly of the virus within the host cell • Release of the virus into the extracellular fluid to infect new cells Copyright © 2009 Pearson Education Inc. 22.6 How Does Medical Care Assist The Immune Response? Vaccinations stimulate the development of memory cells. • A vaccine exposes an individual to antigens produced by a disease organism to stimulate an immune response. • Vaccines often consist of weakened or killed disease microbes or antigens from the disease organism, which are synthesized using genetic engineering. • When the body is exposed to these antigens, it produces swarms of memory cells that confer immunity against living microbes of the same type. Copyright © 2009 Pearson Education Inc. 22.7 What Happens When The Immune System Malfunctions? Allergies are misdirected immune responses. • A foreign substance, such as pollen, enters the bloodstream and is recognized as an antigen by a B cell. • The B cell then proliferates, producing plasma cells that secrete antibodies against the pollen antigen. Copyright © 2009 Pearson Education Inc. 22.7 What Happens When The Immune System Malfunctions? Allergies are misdirected immune responses (continued). • The allergy antibodies bind to the membranes of mast cells of the respiratory and digestive tracts, triggering release of histamine. • Histamine causes leaky capillaries and other symptoms of the immune response. • In the lungs, histamine causes mucus secretion that results in a runny nose, sneezing, and congestion. Copyright © 2009 Pearson Education Inc. 22.7 What Happens When The Immune System Malfunctions? An allergic reaction First exposure to pollen (yellow) stimulates B cells to produce “allergy” plasma cells Plasma cells produce allergy antibodies mast cell plasma cell Allergy antibodies bind to mast cells Reexposure to pollen results in pollen binding to allergy antibodies on mast cells Binding of pollen stimulates mast cells to release histamine (blue), triggering the inflammatory response Fig. 22-14 Copyright © 2009 Pearson Education Inc. 22.7 What Happens When The Immune System Malfunctions? An autoimmune disease is an immune response against the body’s own cells. • Some types of anemia are caused by antibodies that destroy a person’s red blood cells. • Juvenile-onset diabetes begins when the immune system attacks the insulin-secreting cells of the pancreas. • Rheumatoid arthritis results when the immune system attacks cartilage in the joints. • There are no known cures for autoimmune diseases. Copyright © 2009 Pearson Education Inc. 22.7 What Happens When The Immune System Malfunctions? Immune deficiency diseases occur when the body cannot mount an effective immune response against invaders. Copyright © 2009 Pearson Education Inc. 22.7 What Happens When The Immune System Malfunctions? There are two very different disorders in which the immune system cannot combat routine infections—one inherited, the other acquired. • Severe combined immune deficiency (SCID) is a family of genetic defects in which few or no immune cells are formed. • Acquired immune deficiency syndrome (AIDS) is a viral infection that destroys a formerly functional immune system. Copyright © 2009 Pearson Education Inc. 22.7 What Happens When The Immune System Malfunctions? Severe combined immune deficiency • A child with SCID may survive the first few months of life, protected by antibodies from the mother during pregnancy or in her milk. • Once these antibodies are lost, common infections prove fatal because the child is lacking an immune system. • One form of therapy is to transplant bone marrow from a healthy donor so enough immune cells can be produced to confer normal immune responses. Copyright © 2009 Pearson Education Inc. 22.7 What Happens When The Immune System Malfunctions? Acquired immune deficiency syndrome • AIDS is caused by two viruses: human immunodeficiency viruses 1 and 2 (HIV-1 and HIV-2). Fig. 22-15 Copyright © 2009 Pearson Education Inc. 22.7 What Happens When The Immune System Malfunctions? Acquired immune deficiency syndrome (continued) • HIV viruses harm the immune system by destroying the helper T cells that are essential for stimulating both cell-mediated and humoral immune responses. • AIDS does not kill people directly, but rather, makes them susceptible to other diseases as their helper T cell populations decline. Copyright © 2009 Pearson Education Inc. 22.7 What Happens When The Immune System Malfunctions? Acquired immune deficiency syndrome (continued) • HIV can be transmitted only by direct contact with body fluids containing the virus, including blood, semen, vaginal secretions, and breast milk. • The infection can be spread by sexual activity, by sharing needles among drug users, or by blood transfusions. • A woman infected with HIV can transmit the virus to her child during pregnancy, childbirth, or through breast feeding. Copyright © 2009 Pearson Education Inc. 22.7 What Happens When The Immune System Malfunctions? Acquired immune deficiency syndrome (continued) • For persons infected with AIDS, there are two categories of therapy. • First, infections that result from the impaired immune system must be treated as they would be in any patient. • Second, there are drugs that slow the multiplication of HIV and slow the progress of AIDS. • The best solution would be to develop an AIDS vaccine. Copyright © 2009 Pearson Education Inc. 22.7 What Happens When The Immune System Malfunctions? PLAY Animation—The Immune Response Copyright © 2009 Pearson Education Inc. 22.7 What Happens When The Immune System Malfunctions? PLAY Animation—A Detailed View of the HIV Virus Copyright © 2009 Pearson Education Inc. 22.7 What Happens When The Immune System Malfunctions? PLAY Animation—Effects of HIV on the Immune Response Copyright © 2009 Pearson Education Inc. 22.8 How Does The Immune System Combat Cancer? Cancer is a failure of the mechanisms that control the growth of the body’s own cells— the body destroys itself. • Cancer is basically the unchecked growth of malignant tumor cells. • A tumor is a population of cells that has escaped the normal regulatory processes and grows at an abnormal rate. Copyright © 2009 Pearson Education Inc. 22.8 How Does The Immune System Combat Cancer? Most cancerous cells are recognized as foreign. • Cancer cells normally are produced every day, but natural killer cells and cytotoxic T cells screen the body for them and destroy most of them before they can proliferate and spread. • The processes that cause cancer often cause new and slightly different proteins to appear on the surfaces of cancer cells. Copyright © 2009 Pearson Education Inc. 22.8 How Does The Immune System Combat Cancer? Natural killer and cytotoxic cells recognize these as “non-self” antigens and destroy the cells. cytotoxic T cell cancer cell Fig. 22-16 Copyright © 2009 Pearson Education Inc. 22.8 How Does The Immune System Combat Cancer? Most cancerous cells are recognized as foreign (continued). • Some cancer cells evade detection because they do not bear antigens that allow the immune system to recognize them as foreign. • Cancers such as leukemia suppress the immune system. • Others grow so fast that the immune system can’t keep up. Copyright © 2009 Pearson Education Inc. 22.8 How Does The Immune System Combat Cancer? Treatments for cancer depend on distinguishing and selectively killing cancerous cells. • Surgery may not remove all of the cancer if it has started to spread throughout the body. • Radiation can be used to kill small clusters of cancer cells that surgery cannot find, but the whole body cannot be radiated without dangerous effects. • Chemotherapy prevents normal cell division and can result in damage to dividing hair follicle cells and intestinal cells. Copyright © 2009 Pearson Education Inc.