Chapter 38- Immune System First Line of Defense: Barriers Nonspecific defenses, such as the skin and mucous membranes, are barriers to potential pathogens. In addition to being a physical barrier to pathogens, skin also produces sweat, oils, and waxes, which are toxic to many bacteria. A mucous membrane is a layer of
epithelial tissue that covers internal surfaces of the body and secretes mucus, a sticky fluid that traps pathogens. First Line of Defense: Lysozyme Lysozyme is an enzyme found in saliva, tears, and other secretions. It is responsible for breaking down the polysaccharide
walls of many kinds of bacteria and thus it provides some protection against infection. Second Line of Defense: Nonspecific Immunity If a pathogen gets past the skin and the mucous membranes, the body triggers the second line of defense, which is nonspecific immunity. Nonspecific immunity works in
the same way against any pathogen. Nonspecific immunity includes the inflammatory response, the temperature response, and certain proteins. Inflammatory Response An inflammatory response is a series of events that suppress
infection and speed recovery. When cells are damaged they release chemical messengers, such as histamine. Histamine is a substance that increases blood flow to the injured area and increases the permeability of surrounding capillaries. Inflammatory Response
Fluids and white blood cells called phagocytes leak through capillary walls to fight any pathogens that may have entered the body. Phagocytes are cells that ingest and destroy foreign matter, such as microorganisms.
Inflammatory Response, The most common phagocyte in the body is called a neutrophil. Inflammatory Response A macrophage is a white blood cell that
engulfs pathogens and other materials. Another type of white blood cell that attacks pathogen-infected cells is called a natural killer cell. Natural killer cell: A cell that can react against and destroy another cell without prior sensitization to it. Natural killer
(NK) cells are part of our defense against cancer cells and virus-infected cells. Inflammatory Response Temperature Response When the body begins to fight pathogens, body temperature may increase. This rise in
temperature is called a fever. The body triggers a fever in order to slow bacterial growth or to promote white blood cell activity. Though a moderate fever can be helpful, high fevers can be dangerous Second Line of Defense:Proteins
Proteins also provide nonspecific defenses. The complement system is a system of proteins that circulate in the bloodstream and become active when they encounter certain pathogens. Interferon is a protein released by cells infected with viruses that enables nearby cells to resist viral infection.
Specific Defense: The Immune System The immune system includes the cells and tissues that recognize and attack foreign substances in the body. The components of the immune system are found throughout
the body. The Immune System Bone marrow, the thymus, lymph nodes, the spleen, adenoids, and tonsils are all part of the immune system. Each part of the immune system plays a special role in defending the body against pathogens. The specialized cells of the immune
system are called lymphocytes. Lymphocytes are white blood cells that exists in two primary forms: T and B cells. The Immune System B cells are white blood cells that are made in the bone marrow and complete their development there or in the spleen. B cells make antibodies. T cells are cells that are made in the bone marrow but complete their development only after traveling to the thymus. T cells also
participate in many immune reactions. Recognizing Pathogens Lymphocytes can provide specific defenses because they recognize pathogens by the antigens on their surface. An antigen is any substance that the immune system does not recognize as part of the body.
Recognizing Pathogens Because the lymphocytes do not recognize the antigen, they start a specific attack known as an immune response. Lymphocytes recognize a pathogen with molecules on their surface called receptor proteins.
Recognizing Pathogens An antigen has a complementary threedimensional shape that allows the receptor protein to bind to it. This is how the lymphocyte recognizes the antigen. Only the specific receptor protein that is complementary to the
antigen will be able to bind there. Immune Response An immune response is a two-part assault on a pathogen. Both parts occur at the same time and require a specialized lymphocyte called a helper T cell. Immune Response The two parts of the immune response are
the cell-mediated immune response and the humoral immune response. T cells activate certain proteins that affect the behavior of other immune cells. These proteins are called cytokines. Cell-Mediated Immune Response Cell-Mediated Immune Response In the cell-mediated immune response, cytokines activate more helper T cells and another type of T cell called a cytotoxic
T cell. Cytotoxic T cells recognize and destroy cells that have been infected by a pathogen. Cell-Mediated Immune Response Also produced during the cell-mediated immune response is a type of T cell called the suppressor T cell.
Suppressor T cells are thought to shut down the immune response after the pathogen has been cleared from the body. Humoral Immune Response The humoral immune response involves the
action of B cells and occurs when antibodies are activated within body fluids. During the humoral response, cytokines stimulate B cells that have receptors that are complementary to the antigen to divide and change.
Immune Response Most B cells form plasma cells. A plasma cell is a white blood cell that produces antibodies. Antibodies are defensive proteins that react to a specific antigen or inactivate or indirectly destroy toxins. Primary and Secondary Immune Responses The first time the body encounters an antigen, the
immune response is called a primary immune response. During this first encounter, the immune system fights off the disease. After the disease is overcome, the immune system creates memory cells. Memory cells are a B cell or T cell that will recognize and attack the antigen or invading cell during subsequent infections. Primary and Secondary Immune Responses The second time the body encounters an antigen,
a secondary immune response occurs. During a secondary immune response, the immune response is faster and more powerful. Most of the time, the secondary immune response protects the body from reinfection by a pathogen. Cell-Mediated Response Humoral-Immune Response
Immunity and Vaccination Immunity is the ability to resist an infectious disease. Immunity can come about in two ways: surviving an initial infection or through vaccination. Vaccination is the introduction of antigens into the body to cause immunity. Vaccines
Vaccination usually involves an injection of a vaccine. A vaccine is a solution that contains a dead or weakened pathogen or material from a pathogen that still contains antigens. The immune system will produce a primary immune response to the antigens. Memory cells can then provide a quick secondary immune response if the antigen ever enters the body again.
Problems of the Immune System Sometimes the immune system can react to harmless antigens. Three examples of this are allergies, asthma, and autoimmune diseases. Allergies An allergy is a physical response to an antigen, which can be a common
substance that produces little or no response in the general population. Asthma Allergies can trigger asthma. Asthma is a respiratory disorder that causes the bronchioles to narrow due to an overreaction to substances
in the air. During an asthma attack, the lining of the bronchioles and other respiratory tissues may also swell and become inflamed. Autoimmune Diseases An autoimmune disease occurs when the immune system
attacks an organisms own cells. Autoimmune diseases can affect organs and tissues in various areas of the body. The Course of HIV Infection AIDS (acquired immunodeficiency syndrome) is a disease caused by HIV (human immunodeficiency virus). Infection by HIV causes the immune system to lose
its ability to fight off pathogens and cancers. HIV infection usually progresses to AIDS in three phases. The Course of HIV Infection Phase I Phase I of HIV infection is called the asymptomatic stage, because there are few
or no symptoms of the disease. During this period of the disease the viruses increase rapidly due to replication. Phase I can last up to 10 years or more. The Course of HIV Infection Phase II The beginning or worsening of symptoms marks the start of the second phase of
HIV infection. B cells continue making antibodies, but T cells begin to decline steadily as the virus continues to replicate. The Course of HIV Infection Phase III Phase III of HIV infection is the point where the number of helper T cells drops so low that they can
no longer stimulate B cells and cytotoxic T cells to fight invaders. AIDS is diagnosed when the helper T-cell count drops below a certain level. The Course of HIV Infection Phase III, continued AIDS is also diagnosed if an opportunistic infection has
developed. Opportunistic infections are illnesses caused by pathogens that produce disease in people with weakened immune systems. These organisms usually do not create problems in people with a healthy immune system. Drug therapy can slow the progress of HIV infection to AIDS,
but there is no cure for AIDS. Transmission of HIV HIV is transmitted by the transfer of body fluids containing HIV or HIV-infected cells. This most commonly occurs by sexual contact with an infected person or by use of syringes and hypodermic needles that
have been contaminated with blood containing HIV. HIV cannot be transmitted through casual contact. Vaccines and Treatments Effective treatments and vaccines for HIV are difficult to create, because HIV has a rapid mutation rate.
Treatment is also difficult because HIV quickly becomes resistant to antiviral drugs. Most HIV treatments are expensive and require a complicated multidrug
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