CK-12 Biology I - Honors

by CK-12 Foundation

  Figure 24.7

  After engulfing an antigen, a nave B cell presents the antigen to a mature T cell. The T cell, in turn, releases cytokines that activate the B cell. Once activated, the B cell can produce antibodies to that particular pathogen.

  Plasma Cells and Antibody Production

  Plasma cells are activated B cells that secrete antibodies. They are specialized to act like antibody factories. Antibodies produced by plasma cells circulate in the blood and lymph. Each antibody recognizes and binds to a specific antigen, depending on the plasma cell that produced it and other factors. The binding of an antibody to its matching antigen forms an antigen-antibody complex, as shown in Figure below. An antigen-antibody complex flags a pathogen or foreign cell for destruction by phagocytosis. The liver removes antigen-antibody complexes from the blood and the spleen removes them from the lymph.

  Figure 24.8

  An antibody molecule has an area that fits one particular antigen. This area is where the antigen binds to the antibody, creating an antigen-antibody complex.

  Memory Cells

  Whereas most plasma cells live just a few days, memory cells live much longer. They may even survive for the lifetime of the individual. Memory cells are activated B (or T) cells that 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 in the future. Memory B cells remain in the lymph, ready to produce specific antibodies against the same pathogen if it shows up in body fluids again.

  Cell-Mediated Immune Response

  There are several different types of T cells, including helper, cytotoxic, memory, and regulatory T cells. T cells are responsible for cell-mediated immunity. Cell-mediated immunity involves the destruction of body cells that are infected with pathogens or have become damaged or cancerous.

  T Cell Activation

  The different types of naïve T cells are activated in the same general way. The mechanism is shown in Figure below. It involves B cells or leukocytes such as macrophages. These other cells engulf pathogens in phagocytosis and display parts of the pathogens’ antigens on their surface. The cells are then called antigen-presenting cells. When a naïve T cell encounters one of these cells with an antigen matching its own, it begins the activation process. After T cells are activated, the various types of T cells play different roles in the immune response.

  Figure 24.9

  A nave T cell is activated when it encounters a B cell or macrophage that has engulfed a pathogen and presents the pathogens antigen on its surface.

  Helper T Cells

  Activated helper T cells do not kill pathogens or destroy infected cells, but they are still necessary for the immune response. In fact, they are considered to be the “managers” of the immune response. After activation, helper T cells divide rapidly and secrete cytokines. These chemical signals control the activity of other lymphocytes. As mentioned above, cytokines from helper T cells activate B cells. They also activate other T cells.

  Most activated helper T cells die out once a pathogen has been cleared from the body. However, some helper T cells remain in the lymph as memory cells. These memory cells are ready to produce large numbers of antigen-specific helper T cells if they are exposed to the same antigen again in the future.

  Cytotoxic T Cells

  Helper cells are needed to activate cytotoxic T cells. Activated cytotoxic T cells destroy tumor cells, damaged cells, and cells infected with viruses. They are also involved in the rejection of transplanted organs. 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 carrying their specific antigen. Whenever they encounter the cells, they destroy them. Illustrated in Figure below is how a cytotoxic T cell destroys a body cell infected with viruses. The cytotoxic T cell releases toxins that form pores, or holes, in the infected cell’s membrane. This causes the cell to burst, destroying both the cell and the viruses inside it.

  Figure 24.10

  A cytotoxic T cell releases toxins that destroy an infected body cell and the viruses it contains.

  After cytotoxic T cells bring a viral infection under control, most of the cytotoxic T cells die off. However, some of them remain as memory cells. If the same pathogen tries to infect the body again, the memory cells mount an effective immune response by producing a new army of antigen-specific cytotoxic T cells.

  Regulatory T Cells

  Regulatory T cells shut down cell-mediated immunity toward the end of an immune response. They also try to suppress any T cells that react against self antigens as though they were foreign. This occurs in automimmune diseases, which you will read about in Lesson 24.3.

  Immunity

  Memory B and T cells help protect you from re-infection by pathogens that have infected you 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 is immunity that results from a pathogen stimulating an immune response and leaving you with memory cells for the specific pathogen. This happens when a pathogen infects your body and makes you sick. As long as the memory cells survive, the pathogen will be unlikely to re-infect you and make you sick again. In the case of some pathogens, memory cells and active immunity last for the life of the individual.

  Active immunity can also occur through immunization. Immunization is deliberate exposure of a person to a pathogen in order to provoke an immune response. The purpose of immunization is to prevent actual infections by the pathogen. The pathogen is typically injected. However, only part of a pathogen, a weakened form of the pathogen, or a dead pathogen is used. This provokes an immune response without making you sick. Diseases you have likely been immunized against include measles, mumps, rubella, whooping cough, and chicken pox.

  Passive Immunity

  Passive immunity is humoral immunity that results when antibodies to a specific pathogen are transferred to an individual who has never been exposed to the pathogen before. Passive immunity lasts only as long as the antibodies survive in body fluids, generally between a few days and several months.

  Passive immunity is acquired by a fetus when it receives antibodies from the mother’s blood. It is acquired by an infant when it receives antibodies from the mother’s milk. Older children and adults can acquire passive immunity through injection of antibodies into the blood. Injection of antibodies is sometimes used as treatment for a disease, such as measles, when people have not been immunized against the disease.

  Lesson Summary

  The lymphatic system is a major component of the immune system. It filters pathogens from lymph and produces lymphocytes, which are the key cells in an immune response.

  Antigens are proteins that the immune system recognizes as foreign to the body. They trigger the activation of lymphocytes.

  Activated B cells produce antibodies against a pathogen’s antigens. Long-lasting memory B cells remain in the body to provide immunity to the specific pathogen.

  Activated T cells destroy tumor cells and cells infected with viruses. Memory T cells remain after an infection to provide antigen-specific immunity.

  Immunity is the ability to resist infection by a pathogen. It can occur by having an immune response to a pathogen or receiving antibodies to a pathogen.

  Review Questions

  List three parts of the lymphatic system and their functions.

  What are antigens and how do lymphocytes “recognize” them?

  How do plasma cells form and help fight pathogens?

  Describe how cytotoxic T cells destroy cells infected with viruses.

  What type of immune response would occur if bacteria invaded your lymph? Explain your answer.

  Explain how immunization prevents a disease such as measles.

  If a disease destroyed a person’s helper T cells, how might this affect the immune response?

  Co
mpare and contrast humoral and cell-mediated immune responses.

  Further Reading / Supplemental Links

  Panno, Joseph, Ph.D., Immune System. Facts on File, 2008.

  http://en.wikibooks.org/wiki/Human_Physiology/The_Immune_System

  http://www.acm.uiuc.edu/sigbio/project/lymphatic/index.html

  http://www.cancer.gov/cancertopics/understandingcancer/immunesystem

  http://www.howstuffworks.com/immune-system.htm

  http://www.kidshealth.org/parent/general/body_basics/immune.html

  http://www.kidshealth.org/parent/general/body_basics/spleen_lymphatic.html

  http://www.lymphomation.org/lymphatic.htm

  http://www.niaid.nih.gov/final/immun/immun.htm

  http://www.niaid.nih.gov/publications/immune/the_immune_system.pdf

  http://www.sciencedaily.com/releases/2007/08/070802181939.htm

  http://www.thebody.com/content/art1788.html

  http://en.wikipedia.org

  Vocabulary

  active immunity

  Immunity that results from a pathogen stimulating an immune response and leaving you with memory cells for the specific pathogen.

  antibody

  Large, Y-shaped proteins called immunoglobulins (Ig) that recognize and bind to antigens; produced by activated B cells. In humans (and other mammals) there are five types of immunoglobulins: IgA, IgD, IgE, IgG, and IgM.

  antigen

  Any protein that can trigger an immune response because it is foreign to the body; includes proteins on pathogens, cancer cells, and the cells of transplanted organs.

  antigen receptor

  A receptor molecule on the surface of a lymphocyte that binds to a particular antigen like a lock and key.

  B lymphocytes (B cells)

  Lymphocytes that are produced in the red bone marrow and mature in the red bone marrow; can recognize and respond to specific pathogens.

  cell-mediated immunity

  Involves the destruction of body cells that are infected with pathogens or have become damaged or cancerous.

  cytotoxic T cells

  Cells that destroy tumor cells, damaged cells, and cells infected with viruses.

  helper T cells

  Considered to be the “managers” of the immune response. After activation, helper T cells divide rapidly and secrete cytokines. These chemical signals control the activity of other lymphocytes.

  immunity

  Protection from re-infection by pathogens that have infected you in the past.

  immune response

  The third line of defense; specific to a particular pathogen.

  immunization

  The deliberate exposure of a person to a pathogen in order to provoke an immune response.

  lymph

  Fluid that leaks out of tiny blood vessels, called capillaries, into spaces between cells in tissues.

  lymph nodes

  Small, oval structures located along the lymphatic vessels that act like filters; pathogens filtered out of the lymph at lymph nodes are destroyed by lymphocytes in the nodes.

  lymphatic system

  System that makes lymphocytes; consists of lymphatic organs, lymphatic vessels, lymph, and lymph nodes. Organs of the lymphatic system include the red bone marrow, thymus, spleen, and tonsils.

  lymphatic vessels

  Form a body-wide circulatory system, similar to the arteries and veins of the cardiovascular system; circulate lymph instead of blood.

  lymphocytes

  Type of white blood cells, or leukocytes, primarily involved in the immune response; recognize and help destroy specific foreign invaders in body fluids and cells.

  memory cells

  Memory cells are activated B (or T) cells that retain a “memory” of a specific pathogen long after an infection is over; help launch a rapid response against the pathogen if it invades the body in the future.

  passive immunity

  A humoral immunity that results when antibodies to a specific pathogen are transferred to an individual who has never been exposed to the pathogen before.

  phagocytosis

  The process of engulfing and breaking down pathogens and other unwanted substances.

  plasma cells

  Activated B cells that secrete antibodies.

  red bone marrow

  Found inside many bones, including the hip, breast, and skull bones; produces leukocytes.

  regulatory T cells

  T cells that shut down cell-mediated immunity toward the end of an immune response; also try to suppress any T cells that react against self antigens as though they were foreign.

  spleen

  A gland in the upper abdomen; filters blood and destroys worn-out red blood cells. Lymphocytes in the spleen destroy any pathogens filtered out of the blood.

  T lymphocytes (T cells)

  Lymphocytes that are produced in the red bone marrow and mature in the thymus; can recognize and respond to specific pathogens; includes helper, cytotoxic, memory, and regulatory T cells.

  thymus

  A gland located in the upper chest behind the breast bone; stores and matures lymphocytes.

  tonsils

  Glands on either side of the pharynx in the throat; traps pathogens, which are then destroyed by lymphocytes in the tonsils.

  Points to Consider

  Sometimes the immune system makes mistakes and things go wrong. What if the immune system responded to a harmless allergen as though it were a deadly pathogen?

  What if the immune system responded to normal body cells as though they were foreign invaders?

  What if pathogens attacked and destroyed cells of the immune system itself? Would it still be able to function?

  Lesson 24.3: Immune System Diseases

  Lesson Objectives

  Explain how allergies occur and list common allergens.

  Describe how autoimmune diseases affect the body.

  Define immunodeficiency and identify ways it can be acquired.

  Explain how HIV is transmitted and how it causes AIDS.

  Introduction

  The immune system usually protects you from pathogens and keeps you well. However, like any other body system, the immune system can malfunction or become diseased. Sometimes the immune system responds to harmless foreign substances as though they were pathogens. Sometimes it mistakes self for nonself and launches an attack against the body’s own cells. Certain diseases can also attack and damage the immune system so it loses the 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 allergic reaction is called an allergen. You can be exposed to allergens by inhaling or ingesting them or by having direct skin contact with them.

  Allergies can vary greatly from person to person. Some people are allergic to many allergens, others to few or none. A tendency to develop allergies can be inherited, so if your mom or dad has allergies, you are more likely to have them as well. Allergy symptoms may be mild or severe. They may develop immediately after exposure to an allergen or not until several days after exposure.

  Severity of Allergies

  Allergy symptoms are caused by the release of histamines, the chemicals that also stimulate inflammation. The symptoms range from scarcely noticeable to potentially fatal. Typical symptoms of mild allergies include itchy eyes, sneezing, and skin rashes. These symptoms may be uncomfortable, but they are not life threatening. Mild allergy symptoms are often treated with antihistamines. Antihistamines are drugs that reduce or eliminate the effects of histamines.

  Immunotherapy, commonly called “allergy shots,” is sometimes recommended for more severe allergies. A person with an allergy is injected with larger and larger amounts of the offending allergen over a period of months or years. This gradually desensitizes the person’s immune system to the allergen. Rather than just treating the symptoms of the allergy, immunotherapy reduces the severity of the all
ergy or eliminates the allergy altogether.

  The most severe allergic reaction is anaphylaxis. Anaphylaxis is an allergic response in which there is a sudden, massive release of histamines throughout the body. This causes collapse of the circulatory system and severe constriction of the breathing passages. Without emergency treatment, anaphylaxis is likely to be fatal. Treatment is usually injection of epinephrine. Epinephrine is the “fight-or-flight” hormone that your adrenal glands normally produce when you are in danger. The hormone suppresses non-emergency body processes, including the immune response.

  Immediate Hypersensitivity Reaction

  When exposure to an antigen causes immediate allergy symptoms, the response is called an immediate hypersensitivity reaction. This is a humoral immune response. Examples of allergens that cause this type of reaction include pollens, bee stings, and peanuts. Anaphylaxis may occur if the allergy is severe.