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Transcript
Third Line of Defence • Aims: • Must be able to state the substances involved in the third line of immunity. • Should be able to describe the production and differentiation of B and T cells. • Could be able to outline the role of B and T cells. The Third Line of Defense • Specific resistance is a third line of defense. The 2nd line of defense The 3rd line of defense • Forms the immune response and targets specific pathogens. • Specialized cells of the immune system, called lymphocytes are: – B-cells: produce specific proteins called antibodies, which are produced against specific antigens. B cell: Antibody production Lymphocytes T cell: Cell-mediated immunity – T-cells: target pathogens directly. 2 Third Line of Defense • There are two main components of the vertebrate immune system: • The humoral immune system involves the action of B-cells, which produce antibodies. The humoral system is associated with serum, the non-cellular part of the blood. • The cell-mediated immune system is associated with the production of specialized lymphocytes called T-cells. • The humoral and cell-mediated systems work separately and together to protect us from disease. Cells of the Third Line of Defense • Two main groups of lymphocytes are involved in specific immunity. • All lymphocytes are produced in the bone marrow. 1. Mature in the bone marrow - B lymphocytes or B cells. 2. Mature in thymus gland - T lymphocytes or T cells. • There are different kinds of B and T cells… Mast Cell - Secretes hisatmines phagocytes Distinguishing Self • Achieved through the Major Histocompatibility Complex (MHC). • The MHC = cluster of tightly linked genes on chromosome 6 in humans. • Genes code for protein molecules (MHC antigens) attached to the surface of body cells. • The MHC antigens are used by the immune system to recognise its own and foreign material. • Class I MHC antigens located on surface of all human cells. • Class II MHC antigens restricted to macrophages and B-cells. Antigens • Antigens = foreign substances that cause formation of antibodies. • Antigens include: – Potentially damaging microbes and their toxins. – Substances such as pollen and flea and dust mite feces. – Blood cell surface proteins. – The surface proteins of transplanted tissues and organs. B–Cells • B-cells (B-lymphocytes) originate and mature in the bone marrow of the long bones (e.g. the femur). • Migrate from the bone marrow to the lymphatic organs. • B-cells defend against: – Bacteria and viruses outside the cell – Toxins produced by bacteria (free antigens) • Each B-cell can produce antibodies against only one specific antigen. • A mature B-cell may carry up to 100 000 antibody molecules embedded in its surface membrane. B-Cells • Each B-Cell only has one type of antibody… • The body needs to defend itself against millions of antigens. • Only a small number of each corresponding antibody. • The type of antibody is genetically predetermined. • Having a small number of each antibody type means, more chance of ‘recognising’ the antigen. Humoral Immunity Other B-cells recognize different antigens Surface antigen The humoral response begins when a foreign protein (antigen) activates a particular B-cell. Recognition B-cell The particular B-cells multiply, to form many plasma cells. Plasma cells make antibodies specifically designed to attack and kill the identified pathogen. Some B-cells differentiate into long lived memory cells. These memory cells will rapidly produce antibodies if the same pathogen enters the body again. Pathogen Plasma cells Second Exposure Antibodies Original B-cell B–Cell Differentiation • B-cells differentiate into two kinds of cells: Memory cell • Memory cells: When these encounter the same antigen again (even years or decades after the initial infection), they rapidly differentiate into antibody-producing plasma cells. • Plasma cells: These secrete antibodies against antigens. Each plasma cell lives for only a few days, but can produce about 2000 antibody molecules per second. Antibody Plasma cell After the Antigen has Attached to an Immunoglobulin… • The selected B-Cell then needs to reproduce rapidly so that there is a large number of identical cells that can respond to the antigen. • This is called clonal expansion. • Cells cloned in this way have exactly the same genetic material and immunoglobulins. • Most of these cells will then differentiate into plasma cells… Antigens and Antibodies Molecular model • Antibodies (immunoglobulins) are proteins made in response to antigens. – Antibodies recognize and bind to antigens. – Antibodies are highly specific and can help destroy antigens. – Each antibody has at least two sites that can bind to an antigen. Symbolic model Antibody One of the two binding sites on the antibody Antigen Antibody Structure 16 Immunoglobulins • Although there are over 10 million different types of immunoglobulins in our bodies, they exist in several classes: • IgM antibodies are usually the first to be secreted – cause an agglutination of antigens (makes easier for phagocytes). • IgG activate complement proteins,neutralise toxins directly. • IgA neutralise pathogens in the digestive, respiratory and reproductive tracts. • IgE help initiate inflammation after infection. 17 Inactivation of Antigens Neutralization Clumping particulate antigens Precipitation of soluble antigens Antibodies Antibody Virus Bacterial cell Soluble antigens Toxin Antibodies bind to viral binding sites and coat bacterial toxins. Soluble antigens are stuck together to form precipitates. Solid antigens such as bacteria are stuck together in clumps. Enhances Phagocytosis Macrophage Bacteria Inactivation of Antigens Activation of complement Complement Bacterial cell Tags foreign cells for destruction by phagocytes. Enhances phagocytosis Enhances inflammation Leads to rupture of cell Blood vessel Lesion Macrophage Macrophage Bacteria Bacteria Bacterial cell Activity • Complete the activities on pages 155 and 156 in Biozone book. T-Cells • T-cells originate from stem cells and mature after passing through the thymus gland. • They respond only to antigenic fragments that have been processed and presented bound to the MHC by infected cells or macrophages (phagocytic cells). • T-cells defend against: – Intracellular bacteria and viruses. – Protozoa, fungi, flatworms, and roundworms. – Cancerous cells and transplanted foreign tissue. T-Cell Differentiation • T-cells can differentiate into four specialized types of cell: – Helper T-cell •Activates cytotoxic T cells and other helper T cells. •Necessary for B-cell activation. – Suppressor T-cell •Regulates immune response by turning it off when no more antigen is present. – T-cell for delayed hypersensitivity •Causes inflammation in allergic reactions and rejection of tissue transplants. – Cytotoxic (Killer) T-cell •Destroys target cells on contact. Cell Mediated Immunity Antigens, such as those produced by abnormal cells, are identified by and activate specific killer T-cells. Killer T-cells Antigen produced by abnormal cell Recognitio n Helper T-cell Note: HIV (the AIDS virus) disrupts the cellular immune system by destroying helper T-cells. The killer T-cells attach to and destroy the abnormal cell. Killer T-cells remain as memory cells to quickly attack any abnormal cells that reappear. With the assistance of helper T-cells the killer T-cells begin to multiply. Activity • Complete the activities on pages 153 in Biozone book.