Introductory Microbiology BIOL 228 Lecture 15 Immunology PDF

Introductory Microbiology BIOL 228 Nusrat J. Urmi Biology Dept. , OC © 2016 Pearson Education, Inc.  Lecture 15 Immunology...

Introductory Microbiology BIOL 228 Nusrat J. Urmi Biology Dept. , OC © 2016 Pearson Education, Inc.  Lecture 15 Immunology Nusrat J. Urmi Biology Dept. , OC © 2016 Pearson Education, Inc. Key Concept 5: In adaptive immunity, receptors provide pathogen-specific recognition Adaptive Immunity Whereas innate immunity is characterized by broadly targeted responses triggered by common structural features on microorganisms, Adaptive immunity is directed toward specific molecular components of the microbes (their antigens). Classes of Adaptive Immunity Classes of adaptive immunity: Active and Passive – The immune response may be active, which is generated from exposure to an antigen or passive, which is the transfer of antibodies or immune cells Active immunity develops memory cells, therefore producing a long-lasting immunity Passive immunity consists of pre-formed antibodies or cells and has a rapid effect, but doesn’t confer lasting immunity Classes of Adaptive Immunity Classes of adaptive immunity: Active and Passive – Natural immunity occurs without direct medical intervention, while artificial immunity requires injections or infusions natural active immunity: getting a disease and recovering natural passive immunity: passing antibodies to nursing infants through breast milk artificial active immunity: receiving a vaccination shot and developing immunity artificial passive immunity: receiving pre-formed antibodies (antiserum) Adaptive Immunity The adaptive response relies on two types of lymphocytes, or white blood cells. Lymphocytes that mature in the thymus above the heart are called T cells. Lymphocytes that mature in bone marrow are called B cells. Antigens are substances that can elicit a response from B or T cells. T or B cells bind to antigens via antigen receptors specific to that pathogen. The small accessible part of an antigen that binds to antigen receptor is called an epitope. Adaptive Immunity Each individual B or T cell is specialized to recognize a specific type of molecule. The antigen receptors of B and T cells have similar components, but encounter antigens in different ways. Figure 43.8 B and T lymphocytes. Antigen Recognition by B Cells and Antibodies Each B cell antigen receptor is a Y-shaped molecule with two identical heavy chains and two identical light chains. Constant regions of the chains vary little among B cells, whereas variable regions differ greatly. The variable regions provide antigen specificity. Figure 43.9 The structure of a B cell antigen receptor. Antigen Recognition by B Cells and Antibodies Binding of a B cell antigen receptor to an antigen is an early step in B cell activation. B cell activation gives rise to cells that secrete a soluble form of the receptor called an antibody or immunoglobulin (Ig). Different antibodies can recognize distinct epitopes on the same antigen. Furthermore, antibodies can recognize free antigens as well as antigens on a pathogen’s surface. Figure 43.10 Antigen recognition by B cells and antibodies. Antigen Recognition by T Cells Each T cell receptor consists of two different polypeptide chains (called  and ). T cells bind to antigen fragments displayed or presented on a host cell. These antigen fragments are bound to cell-surface proteins called MHC (major histocompatibility complex) molecules. Figure 43.11 The structure of a T cell antigen receptor. Antigen Recognition by T Cells In infected cells, MHC molecules bind and transport antigen fragments to the cell surface, a process called antigen presentation. Class I MHC molecules, found on almost all nucleated cells of the body. Class II MHC molecules are made by dendritic cells, macrophages, and B cells. A T cell can then bind both the antigen fragment and the MHC molecule. This interaction is necessary for the T cell to participate in the adaptive immune response. Figure 43.12 Antigen recognition by T cells. Proliferation of B Cells and T Cells Once activated by infection, a B or T cell undergoes multiple cell divisions (called clonal selection) to produce a clone of identical cells. Two types of clones are produced: 1. short-lived activated effector cells/ plasma cells that secrete antibodies & act immediately against the antigen. 2. long-lived memory cells that can give rise to effector cells if the same antigen is encountered again. Figure 43.14 Clonal selection of B cells. Immunological Memory Immunological memory is responsible for long-term protections against diseases, due to a prior infection or vaccination. The first exposure to a specific antigen represents the primary immune response. During this time, selected B and T cells give rise to their effector forms. In the secondary immune response, memory cells facilitate a faster, more efficient response. Figure 43.15 The specificity of immunological memory. Key Concept 6: Adaptive immunity defends against infection of body fluids and body cells Humoral vs Cell-mediated immune response Adaptive immunity can be divided into: Humoral immune response: B cells are involved in humoral immunity. Antibodies secreted by B cells help neutralize or eliminate toxins and pathogens in the blood and lymph. Cell-mediated immune response: Specialized T cells (cytotoxic T cells) are involved in cell-mediated immunity and destroy affected host cells. Helper T Cells Helper T cells trigger both humoral and cell-mediated immune responses. Signals from helper T cells initiate production of antibodies and activate T cells that kill infected cells. Figure 43.17 The central role of helper T cells in humoral and cell-mediated immune responses. Helper T Cells An antigen-presenting cell displays antigen fragments complexed with class-II MHC molecules on the cell surface. A specific helper T cell binds to this complex via its antigen receptor. Binding of the helper T cell promotes secretion of cytokines by the antigen presenting cell. These cytokines, along with cytokines from the helper T cell itself, activate the helper T cell and proliferates and forms a clone of helper T cells. This clone of activated helper T cells secrete other cytokines, which help activate B cells and cytotoxic T cells. Figure 43.17 The central role of helper T cells in humoral and cell-mediated immune responses. Activation of B Cells In response to cytokines from helper T cells and an antigen, a B cell proliferates and differentiates into memory B cells and plasma cells. Plasma cells begin producing and secreting antibodies. Figure 43.18 Activation of a B cell in the humoral immune response. Antibody Function Antibodies do not kill pathogens; instead they mark pathogens for destruction. Figure 43.19 A. In neutralization, antibodies bind to viral Neutralization surface proteins preventing infection of a host cell. B. In opsonization, antibodies bind to antigens on bacteria, triggering phagocytosis. Figure 43.20 Opsonization C. Antigen-antibody complexes may bind to a complement protein, which triggers a cascade of complement protein activation. Ultimately a membrane attack complex forms a pore in foreign cell’s membrane, leading to its lysis. Figure 43.21 Activation of complement system and pore formation. Cytotoxic T Cells Cytotoxic T cells are effector cells in the cell-mediated immune response. They use toxic proteins to kill cells infected by viruses or other intracellular pathogens. They recognize fragments of foreign proteins produced by infected cells and binds to class I MHC molecules. Activated cytotoxic T cells secrete proteins that disrupt the membranes of target cells and triggers apoptosis. Figure 43.22 The killing action of cytotoxic T cells on an infected host cell. Summary of the Humoral and Cell-Mediated Immune Responses Both the humoral and cell-mediated responses can include primary and secondary immune responses. Memory cells enable the secondary response. Figure 43.23 An overview of the adaptive immune response.