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    Biology Lesson 17: The Immune System and Disease
    Does this organism look like a space alien? A scary creature from a nightmare? In fact its a 1-cm long worm that lives in the human body and causes serious harm. It enters the body through a hair follicle of the skin when its in a much smaller stage of its life cycle.
    Like this worm many other organisms can make us sick if they manage to enter our body. Fortunately for us our immune system is able to keep out most such invaders. When you read this chapter youll learn how your immune system keeps you safe from harmincluding from scary creatures like this!
    Section 1: Nonspecific Defenses
    Section Objectives
    Vocabulary
    Introduction
    The immune system protects the body from worms germs and other agents of harm. The immune system is like a medieval castle. The outside of the castle was protected by a moat and high stone walls. Inside the castle soldiers were ready to fight off any invaders that managed to get through the outer defenses. Like a medieval castle the immune system has a series of defenses. In fact it has three lines of defense. Only pathogens that are able to get through all three lines of defense can harm the body.
    The First Line of Defense
    The bodys first line of defense consists of different types of barriers that keep most pathogens out of the body. Pathogens are disease-causing agents such as bacteria and viruses. These and other types of pathogens are described in Figure below (image in .pdf file). Regardless of the type of pathogen however the first line of defense is always the same.
    Types of pathogens that commonly cause human diseases include bacteria viruses fungi and protozoa. Which type of pathogen causes the common cold?
    (image in .pdf file)
    Mechanical Barriers
    Mechanical barriers physically block pathogens from entering the body. The skin is the most important mechanical barrier. In fact it is the single most important defense the body has. The outer layer of the skin is tough and very difficult for pathogens to penetrate. Mucous membranes provide a mechanical barrier at body openings. They also line the respiratory GI urinary and reproductive tracts. Mucous membranes secrete mucus a slimy substance that traps pathogens. The membranes also have hair-like cilia. The cilia sweep mucus and pathogens toward body openings where they can be removed from the body. When you sneeze or cough pathogens are removed from the nose and throat (see Figure below) (image in .pdf file). Tears wash pathogens from the eyes and urine flushes pathogens out of the urinary tract.
    A sneeze can expel many pathogens from the respiratory tract.
    (image in .pdf file)
    Chemical Barriers
    Chemical barriers destroy pathogens on the outer body surface at body openings and on inner body linings. Sweat mucus tears and saliva all contain enzymes that kill pathogens. Urine is too acidic for many pathogens and semen contains zinc which most pathogens cannot tolerate. In addition stomach acid kills pathogens that enter the GI tract in food or water.
    Biological Barriers
    Biological barriers are living organisms that help protect the body. Millions of harmless bacteria live on the human skin. Many more live in the GI tract. The harmless bacteria use up food and space so harmful bacteria cannot grow.
    The Second Line of Defense
    If you have a cut on your hand the break in the skin provides a way for pathogens to enter your body. Assume bacteria enter through the cut and infect the wound. These bacteria would then encounter the bodys second line of defense.
    Inflammatory Response
    The cut on your hand may become red warm and swollen. These are signs of an inflammatory response. This is the first reaction of the body to tissue damage or infection. As explained in Figure below (image in .pdf file) the response is triggered by chemicals called cytokines and histamines which are released when tissue is injured or infected. The chemicals communicate with other cells and coordinate the inflammatory response. You can see an animation of the inflammatory response at:
    http://www.sumanasinc.com/webcontent/animations/content/inflammatory.html
    This drawing shows what happens during the inflammatory response. Why are changes in capillaries important for this response?
    (image in .pdf file)
    The inflammatory response is discussed at:
    http://www.youtube.com/user/khanacademy#p/c/7A9646BC5110CF64/56/FXSuEIMrPQk
    Leukocytes
    The chemicals that trigger an inflammatory response attract leukocytes to the site of injury or infection. Leukocytes are white blood cells. Their role is to fight infections and get rid of debris. Leukocytes may respond with either a nonspecific or a specific defense.
    In this image leukocytes (white) are attacking pathogens (star-shaped).
    (image in .pdf file)
    A summary of the nonspecific defenses can be viewed at:
    http://www.youtube.com/user/khanacademy#p/c/7A9646BC5110CF64/49/O1N2rENXq_Y
    Section Summary
    Extra Practice
    1. Jera cut her finger. The next day the skin around the cut had become red and warm. Why are these signs of infection?
    2. Explain how the inflammatory response helps fight an infection.
    Points to Consider
    The bodys first and second lines of defense are the same regardless of the particular pathogen involved. The bodys third line of defense is different. It tailors the response to the specific pathogen.
    Section 2: The Immune Response
    Section Objectives
    Vocabulary
    Introduction
    Like the immune systems of other vertebrates the human immune system is adaptive. If pathogens manage to get through the bodys first two lines of defense the third line of defense takes over. The third line of defense is referred to as the immune response.This defense is specific to a particular pathogen and it allows the immune system to remember the pathogen after the infection is over. If the pathogen tries to invade the body again the immune response against that pathogen will be much faster and stronger.
    You can watch an overview of the immune response at this link: http://www.youtube.com/watch?v=G7rQuFZxVQQ&feature=related.
    The types of immune responses is discussed at http://www.youtube.com/user/khanacademy#p/c/7A9646BC5110CF64/50/rp7T4IItbtM.
    Lymphatic System
    The immune response mainly involves the lymphatic system. The lymphatic system is a major part of the immune system. It produces leukocytes called lymphocytes. Lymphocytes are the key cells involved in the immune response. They recognize and help destroy particular pathogens in body fluids and cells. They also destroy certain cancer cells.
    You can watch an animation of the lymphatic system at:

    Structures of the Lymphatic System
    Figure below (image in .pdf file) shows the structures of the lymphatic system. They include organs lymph vessels lymph and lymph nodes. Organs of the lymphatic system are the bone marrow thymus spleen and tonsils.
    The lymphatic system consists of organs vessels and lymph.
    (image in .pdf file)
    Lymphatic Vessels and Lymph
    Lymphatic vessels make up a body-wide circulatory system. The fluid they circulate is lymph. Lymph is a fluid that leaks out of capillaries into spaces between cells. As the lymph accumulates between cells it diffuses into tiny lymphatic vessels. The lymph then moves through the lymphatic system from smaller to larger vessels. It finally drains back into the bloodstream in the chest. As lymph passes through the lymphatic vessels pathogens are filtered out at small structures called lymph nodes (see Figureabove) (image in .pdf file).
    The filtered pathogens are destroyed by lymphocytes.
    (image in .pdf file)
    Lymphocytes
    The human body has as many as two trillion lymphocytes and lymphocytes make up about 25% of all leukocytes. The majority of lymphocytes are found in the lymphatic system where they are most likely to encounter pathogens. The rest are found in the blood. There are two major types of lymphocytes called B cells and T cells. These cells get their names from the organs in which they mature. B cells mature in bone marrow and T cells mature in the thymus. Both B and T cells recognize and respond to particular pathogens.
    Antigen Recognition
    B and T cells actually recognize and respond to antigens on pathogens. Antigens are molecules that the immune system recognizes as foreign to the body. Antigens are also found on cancer cells and the cells of transplanted organs. They trigger the immune system to react against the cells that carry them. This is why a transplanted organ may be rejected by the recipients immune system. How do B and T cells recognize specific antigens? They have receptor molecules on their surface that bind only with particular antigens. As shown in Figure below (image in .pdf file) the fit between an antigen and a matching receptor molecule is like a key in a lock.
    An antigen fits the matching receptor molecule like a key in a lock.
    (image in .pdf file)
    Humoral Immune Response
    There are actually two types of immune responses: humoral and cell-mediated. The latter response is described later in this section. The humoral immune responseinvolves mainly B cells and takes place in blood and lymph. You can watch an animation of the humoral immune response at:
    http://www.cancerresearch.org/resources.aspx?id=586
    B Cell Activation
    B cells must be activated by an antigen before they can fight pathogens. This happens in the sequence of events shown in Figure below (image in .pdf file). First a B cell encounters its matching antigen and engulfs it. The B cell then displays fragments of the antigen on its surface. This attracts a helper T cell (which is further discussed below). The helper T cell binds to the B cell at the antigen site and releases cytokines that tell the B cell to develop into a plasma cell.
    B lymphocytes are further discussed at:

    Activation of a B cell must occur before it can respond to pathogens. What role do T cells play in the activation process?
    (image in .pdf file)
    Plasma Cells and Antibody Production
    Plasma cells are activated B cells that secrete antibodies. Antibodies are large Y-shaped proteins that recognize and bind to antigens. Plasma cells are like antibody factories producing many copies of a single type of antibody. The antibodies travel throughout the body in blood and lymph. Each antibody binds to just one kind of antigen. When it does it forms an antigen-antibody complex (see Figure below) (image in .pdf file). The complex flags the antigen-bearing cell for destruction by phagocytosis.
    The video at the link below shows how this happens.
    http://www.youtube.com/watch?v=lrYlZJiuf18&feature=fvw
    An antibody matches only one type of antigen.
    (image in .pdf file)
    Memory Cells
    Most plasma cells live for just a few days but some of them live much longer. They may even survive for the lifetime of the individual. Long-living plasma cells are called memory cells. They retain a memory of a specific pathogen long after an infection is over. They help launch a rapid response against the pathogen if it invades the body again in the future.
    Cell-Mediated Immune Response
    The other type of immune response the cell-mediated immune response involves mainly T cells. It leads to the destruction of cells that are infected with viruses. Some cancer cells are also destroyed in this way. There are several different types of T cells involved in a cell-mediated immune response including helper cytotoxic and regulatory T cells.
    T Cell Activation
    All three types of T cells must be activated by an antigen before they can fight an infection or cancer. T cell activation is illustrated in Figure below (image in .pdf file). It begins when a B cell or nonspecific leukocyte engulfs a virus and displays its antigens. When the T cell encounters the matching antigen on a leukocyte it becomes activated. What happens next depends on which type of T cell it is.
    T cell activation requires another leukocyte to engulf a virus and display its antigen.
    (image in .pdf file)
    Helper T Cells
    Helper T cells are like the managers of the immune response. They secrete cytokines which activate or control the activities of other lymphocytes. Most helper T cells die out once a pathogen has been cleared from the body but a few remain as memory cells. These memory cells are ready to produce large numbers of antigen-specific helper T cells like themselves if they are exposed to the same antigen in the future.
    Helped T cells are discussed at:
    http://www.youtube.com/user/khanacademy#p/c/7A9646BC5110CF64/53/uwMYpTYsNZM
    Cytotoxic T Cells
    Cytotoxic T cells destroy virus-infected cells and some cancer cells. Once activated a cytotoxic T cell divides rapidly and produces an army of cells identical to itself. These cells travel throughout the body searching for more cells to destroy. Figurebelow (image in .pdf file) shows how a cytotoxic T cell destroys a body cell infected with viruses. The T cell releases toxins that form pores in the membrane of the infected cell. This causes the cell to burst destroying both the cell and the viruses inside it.
    A cytotoxic T cell releases toxins that destroy an infected body cell and the viruses it contains.
    (image in .pdf file)
    After an infection has been brought under control most cytotoxic T cells die off. However a few remain as memory cells. If the same pathogen enters the body again the memory cells mount a rapid immune response. They quickly produce many copies of cytotoxic T cells specific to the antigen of that pathogen.
    Regulatory T Cells
    Regulatory T cells are responsible for ending the cell-mediated immune response after an infection has been curbed. They also suppress T cells that mistakenly react against self antigens. What might happen if these T cells were not suppressed?
    Immunity
    Memory B and T cells help protect the body from re-infection by pathogens that infected the body in the past. Being able to resist a pathogen in this way is called immunity. Immunity can be active or passive.
    Active Immunity
    Active immunity results when an immune response to a pathogen produces memory cells. As long as the memory cells survive the pathogen will be unable to cause a serious infection in the body. Some memory cells last for a lifetime and confer permanent immunity. Active immunity can also result from immunization. Immunizationis the deliberate exposure of a person to a pathogen in order to provoke an immune response and the formation of memory cells specific to that pathogen. The pathogen is often injected. However only part of a pathogen a weakened form of the pathogen or a dead pathogen is typically used. This causes an immune response without making the immunized person sick. This is how you most likely became immune to measles mumps and chicken pox.
    Passive Immunity
    Passive immunity results when antibodies are transferred to a person who has never been exposed to the pathogen. Passive immunity lasts only as long as the antibodies survive in body fluids. This is usually between a few days and a few months. Passive immunity may be acquired by a fetus through its mothers blood. It may also be acquired by an infant though the mothers breast milk. Older children and adults can acquire passive immunity through the injection of antibodies.
    Section Summary
    A review of B cells and T cells is available at:
    http://www.youtube.com/user/khanacademy#p/c/7A9646BC5110CF64/55/xaz5ftvZCyI
    Extra Practice
    1. If a disease destroyed a persons helper T cells how might this affect the ability to launch an immune response?
    2. Compare and contrast humoral and cell-mediated immune responses.
    3. How is active immunity different from passive immunity? Why does active immunity last longer?
    4. Explain how immunization prevents a disease such as measles which is caused by a virus.
    Points to Consider
    Sometimes the immune system makes mistakes and things go wrong.
    Section 3: Immune System Diseases
    Section Objectives
    Vocabulary
    Introduction
    Your immune system usually protects you from pathogens and keeps you well. However like any other body system the immune system itself can develop problems. Sometimes it responds to harmless foreign substances as though they were pathogens. Sometimes it attacks the bodys own cells. Certain diseases can also attack and damage the immune system and interfere with its ability to defend the body.
    Allergies
    An allergy is a disease in which the immune system makes an inflammatory response to a harmless antigen. Any antigen that causes an allergy is called an allergen. Allergens may be inhaled or ingested or they may come into contact with the skin. Two common causes of allergies are shown in Figure below (image in .pdf file). Inhaling ragweed pollen may cause coughing and sneezing. Skin contact with oils in poison ivy may cause an itchy rash.
    Ragweed and poison ivy are common causes of allergies. Are you allergic to these plants?
    (image in .pdf file)
    The symptoms of allergies can range from mild to severe. Mild allergy symptoms are often treated with antihistamines. These are drugs that reduce or eliminate the effects of the histamines that cause allergy symptoms. The most severe allergic reaction is called anaphylaxis. This is a life-threatening response caused by a massive release of histamines. It requires emergency medical treatment.
    You can watch an animated video about how allergic reactions occur and how antihistamines can control them at:
    http://www.youtube.com/watch?v=y3bOgdvV-_M.
    Autoimmune Diseases
    Autoimmune diseases occur when the immune system fails to recognize the bodys own molecules as self or belonging to the person. Instead it attacks body cells as though they were dangerous pathogens. Some relatively common autoimmune diseases are listed in Table below. These diseases cannot be cured although they can be treated to relieve symptoms and prevent some of the long-term damage they cause.
    Autoimmune Diseases
    Name of Disease
    Tissues Attacked by Immune System
    Results of Immune System Attack
    Rheumatoid arthritis
    tissues inside joints
    joint damage and pain
    Type 1 diabetes
    insulin-producing cells of the pancreas
    inability to produce insulin high blood sugar
    Multiple sclerosis
    myelin sheaths of central nervous system neurons
    muscle weakness pain fatigue
    Systemic lupus erythematosus
    joints heart other organs
    joint and organ damage and pain
    Table 24.4 Autoimmune diseases occur when the immune system attacks body cells.
    Why does the immune system attack body cells? In some cases its because of exposure to pathogens that have antigens similar to the bodys own molecules. When this happens the immune system not only attacks the pathogens. It also attacks body cells with the similar molecules.
    Immunodeficiency
    Immunodeficiency occurs when the immune system is not working properly. As a result it cannot fight off pathogens that a normal immune system would be able to resist. Rarely the problem is caused by a defective gene. More often it is acquired during a persons lifetime. Immunodeficiency may occur for a variety of reasons:
    HIV and AIDS
    Human immunodeficiency virus (HIV) is a virus that attacks the immune system. An example of HIV is shown in Figure below (image in .pdf file). Many people infected with HIV eventually develop acquired immune deficiency syndrome (AIDS). This may not occur until many years after the virus first enters the body.
    HIV is a virus that attacks cells of the immune system.
    (image in .pdf file)
    HIV Transmission
    HIV is transmitted or spread through direct contact of mucous membranes or body fluids such as blood semen or breast milk. As shown in Figure below (image in .pdf file) transmission of the virus can occur through sexual contact or the use of contaminated hypodermic needles. It can also be transmitted through an infected mothers blood to her baby during late pregnancy or birth or through breast milk after birth. In the past HIV was also transmitted through blood transfusions. Because donated blood is now screened for HIV the virus is no longer transmitted this way.
    HIV may be transmitted in all of the ways shown here. Based on how HIV is transmitted what can people do to protect themselves from becoming infected? What choices can they make to prevent infection?
    (image in .pdf file)
    HIV and the Immune System
    HIV infects and destroys helper T cells. As shown in Figure below (image in .pdf file) the virus injects its own DNA into a helper T cell and uses the T cells machinery to make copies of itself. In the process the T cell is destroyed and the virus copies go on to infect other helper T cells.
    This diagram shows how HIV infects and destroys T cells.
    (image in .pdf file)
    HIV is able to evade the immune system and keep destroying T cells. This occurs in two ways:
    As time passes the number of HIV copies keeps increasing while the number of helper T cells keeps decreasing. The graph in Figure below (image in .pdf file) shows how the number of T cells typically declines over a period of many years following the initial HIV infection. As the number of T cells decreases so does the ability of the immune system to defend the body. As a result an HIV-infected person develops frequent infections. Medicines can slow down the virus but not get rid of it so there is no cure at present for HIV infections or AIDS. There also is no vaccine to immunize people against HIV infection but scientists are working to develop one.
    It typically takes several years after infection with HIV for the drop in T cells to cripple the immune system. What do you think explains the brief spike in T cells that occurs early in the HIV infection shown here?
    (image in .pdf file)
    AIDS
    AIDS is not a single disease but a set of diseases. It results from years of damage to the immune system by HIV. It occurs when helper T cells fall to a very low level and opportunistic diseases occur (see Figure above) (image in .pdf file). Opportunistic diseases are infections and tumors that are rare except in people with immunodeficiency. The diseases take advantage of the opportunity presented by people whose immune systems cant fight back. Opportunistic diseases are usually the direct cause of death of people with AIDS.
    AIDS and HIV were first identified in 1981. Scientists think that the virus originally infected monkeys but then jumped to human populations probably sometime during the early to mid-1900s. This most likely occurred in West Africa but the virus soon spread around the world (see Figure below) (image in .pdf file). Since then HIV has killed more than 25 million people worldwide. The hardest hit countries are in Africa where medicines to slow down the virus are least available. The worldwide economic toll of HIV and AIDS has also been enormous.
    (image in .pdf file)
    HIV Research: Beyond the Vaccine
    Over the past 15 years the number of people who die of AIDS each year in the United States has dropped by 70 percent. But AIDS remains a serious public health crisis among low-income African-Americans particularly women. And in sub-Saharan Africa the virus killed more than 1.6 million people in 2007. Innovative research approaches could lead to new treatments and possibly a cure for AIDS.
    See http://www.kqed.org/quest/television/hiv-research-beyond-the-vaccine for further information.
    Section Summary
    Points to Consider
    Pathogens such as HIV are not the only cause of human disease. Many other things in our environment can also make us sick.
    Lesson 17 Review Questions
    Directions: Answer each of the following questions.
    1. Identify three types of barriers in the bodys first line of defense. Give an example of each type of barrier.
    2. What is the bodys second life of defense? When does it take effect?
    3. Identify the roles of nonspecific leukocytes in the bodys second line of defense.
    4. State how phagocytosis helps defend the body from pathogens.
    5. What is a nonspecific defense?
    l6. List three parts of the lymphatic system and their functions.
    7. What are antigens and how do lymphocytes recognize them?
    8. How do plasma cells form and how do they help fight pathogens?
    9. Describe one way that cytotoxic T cells destroy cells infected with viruses.
    10. What is immunity? What role do memory cells play in immunity?
    Essay submission: Select 1 Biology topic from this lesson and submit a 3-5 paragraph essay about the topic. Remember to cite your sources!

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