Chapter 17 Adaptive Immunity: Specific Defenses of the Host

Transcription

1 Chapter 17 Adaptive Immunity: Specific Defenses of the Host I. Introduction to Specific Defenses of the Host A. Innate immunity (resistance) - An individual s genetically predetermined resistance to certain diseases. Affected by sex, age, nutritional status, and general health. Discussed in Chap 16 B. Acquired (adaptive) Immunity - Ability of the body to counteract specific microbes or foreign substances. Immunity results from the production of protein antibodies and specialized lymphocytes that target a specific antigen that caused their formation. The outcome is destruction or inactivation of the antigen for current or future exposures. The immune system is required for survival but if misdirected toward self antigens can be very destructive to the host. II. Types of Acquired Immunity - Figure A. Naturally acquired active B. Naturally acquired passive C. Artificially acquired active and vaccination D. Artificially acquired passive and gamma globulin fig III. The Duality of the Immune System A. Mediated by lymphocytes via the lymphatic system fig Fluid circulates between interstitial fluid, lymphatic vessels and nodes, then returned to the blood. Antigens is this fluid are processed by the immune system. B. Humoral (Antibody Mediated ) Immunity 1. Protein Antibodies are Produced by B Cells (B lymphocytes) and secreted into extracellular fluids. a) Defends against bacteria, viruses, and toxins in body fluids. C. Cell mediated by specialized lymphocytes Microbiology 8 Chapter17taft.DOC 1

2 1. Involves T-cells (T lymphocytes) and no antibodies. Act against foreign organisms or tissues. Regulates the activation and proliferation of other immune system cells. a) Most effective against intracellular bacteria and viruses, fungi, multicellular parasites, cancer cells, and transplanted tissue. IV. Antigens and Antibodies A. Antigens - Substances that cause the body to produce specific antibodies and/or sensitized T-cells. 1. Foreign- Not part of body chemistry. Nonself vs self recognition so the immune system does not attack the host s own tissues. 2. Proteins or large polysaccharides with a molecular weight greater than 10,000. Combinations can be antigenic: nucleoproteins, lipoproteins, glycoproteins. Most components of an organism are antigenic. 3. Antibodies are formed against specific regions on the surface of an antigen called antigenic determinant (epitope). Figure Hapten is a low molecular weight compound that cannot induce antibody response by itself but can if combined with another molecule (often a host protein). Example: penicillin. Figure 17.2 B. Antibodies - Proteins made in response to antigens that can only recognize and bind with the antigen that stimulated their formation. 1. Structure - Figure 17.3 Typically Y-shaped with two light and two heavy chains. Variable region provides for specific antigen recognition and binding. 2. Immunoglobulin Classes - Table 17.1 Five classes: a) IgG (80% of blood antibodies), crosses placenta b) IgM (5-10% of serum antibodies),used to detect initial infection c) IgA (primary in mucous and secretions for localized protection), especially important in respiratory and GI tract. d) IgD (antigen receptors on B cells for initiation of immune response) Microbiology 8 Chapter17taft.DOC 2

3 e) IgE (bind to mast cells in tissue), mediates allergies, lysis of parasitic worms. V. B Cells and Humoral Immunity A. Bone marrow stem cells give rise to B cells that migrate to lymphoid organs where they come in contact with antigen, which they recognize, via antibody on surface of their cells. Figures The antigen is processed and presented with a MHC class II site on the surface of the B cell where it can be recognized by a T H cell via its specific receptor. The MHC is a self identity marker that prevents the host from making antibidies harmful to itself. Most antigens elicit this type of response, called a T dependent antigen repsonse. 2. B cells go through a process of producing a clone of plasma cells (antibody producing cells) and memory cells. An individual may recognize about different antigens determined by their genetic makeup. Fig 17.5 B. Antigen-Antibody Binding and Its Results - Figure 17.7 Agglutination, Opsonization, Neutralization, Complement activation, Inflammation, Antibody-dependent cell-mediated cytotoxicity fig C. Immunological Memory Figure Primary: gradual, several days with no antibody, then IgM, then IgG. Peak titers at days. 2. Secondary Immune Response: quick via memory cells that persists for years, primarily IgG. Some antibody already present followed by peak in 2-7 days. 3. T cell memory is similar and necessary for self vs nonself recognition VI. T Cells and Cell-Mediated Immunity A. Chemical Messengers of Immune Cells 1. Cytokines - Way in which cells communicate - Table 17.3 Example: Interleukin 2 (IL2) stimulates antigen stimulated T helper cells and B cells, activates T cytotoxic cells and natural killer cells. Microbiology 8 Chapter17taft.DOC 3

4 B. Cellular Components of Immunity 1. Lymphocytes produced in bone marrow migrate to thymus, where under the influence of thymic hormones, differentiate into T-cells. Fig 17.8 In response to antigen effector T-cells are stimulated to be produced. These cells proliferate and carry out cell mediated immunity. The body s ability to make new T cells decreases with age beginning in late adolescence so the immune system is relatively weak in old age. Memory cells remain to provide protection. 2. Types of T cells - Classified according to function and cell surface receptors (CD). Table 17.2 a) Antigen presenting cell (APC)= macrophages or dendritic cells fig 17.9, fig 17.12, Self molecule = Major histocompatibility complex (MHC) (1) Is not a T cell but is necessary to process antigen to present to T cell (2) A T cell only recognizes the antigen if presented with an MHC self surface marker close to it that is unique to that person defined in their genetic code. The immune system uses this to determine self from nonself. b) T-helper cell (T H,CD4), T-cytotoxic cell (T C,CD8) Figure 17.9 (1) Antigen must be presented by APC to T helper cell. IL1 secreted by APC to stimulate T H cell. (2) Antigen must be closely associated with MHC for successful recognition by T cell. IL 2 secreted by T H cells to stimulate and differentiate more T and B cells. Amplifies response. (3) T C cells attack whole cells (e.g., cancer cells, virus infected cells where antibodies can t get at them) with surface antigen and MHC class I markers. Class I markers are found on nucleated host cells. So, these cells can attack host cells that have an altered surface antigen, such as tumor cells and virus infected cells. Fig Microbiology 8 Chapter17taft.DOC 4

5 c) T R - Turns off (suppresses) the immune response when antigen no longer present. May be involved with regulation of tissue rejection and auto immune reactions. C. Interrelationship of Cell-Mediated and Humoral Immunity - Figure Microbiology 8 Chapter17taft.DOC 5