Immune System PDF

Immune system Dr/ Fahad Alzowahi Assistant professor in Microbiology  Immune system: The collection of cells, tissues, organs and molecules which provide non-specific and specific protection against" Microorganisms, microbial toxins and tumor cells. 1- Lymphoid tissue and organs: Lym...

Immune system Dr/ Fahad Alzowahi Assistant professor in Microbiology  Immune system: The collection of cells, tissues, organs and molecules which provide non-specific and specific protection against" Microorganisms, microbial toxins and tumor cells. 1- Lymphoid tissue and organs: Lymphoid tissue and organs is conveniently divided into the central or primary and peripheral or secon- dary lymphoid organs. A) Central or primary lymphoid organs: a- Bone marrow: fill central cavity of all bones, it is responsible for synthesis of blood cells (hematopoi- esis) and proliferation of blood cells to myeloid or lymphoid cells (figure 12). Fig. (12): Hematopoiesis process. b- Thymus gland: is a small gland lies on chest behind the sternum, represent the site in which T- lymphocyte develop and proliferate to T cytotoxic or T helper cells (figure 13). Fig. (13): Thymus gland. B) Peripheral or secondary lymphoid organs: a. Lymph node. b. Spleen. c. Other lymphoid tissues: Mucosa-associated lymphoid tissues (tonsils, adenoid, Peyer's patches), appendix and lymphatic channels. 2. Molecules of the immune system: a- Antibodies (immunoglobulins), are protein prod- ucts of certain B lymphocytes (plasma cells) with a precise specificity for a particular antigen. b- Lymphokines, are subset of cytokines (protein products) that are secreted by T lymphocyte to direct the immune system response by signaling between its cells (activation of immune response). 3. Cells of the immune system: a- Lymphocytes: 1- B lymphocytes: produced and mature within the bone marrow; when they leave it, each expres-ses a unique antigen-binding receptor on its mem-brane. This antigen-binding or B-cell receptor is a memb- rane bound antibody molecule. B lymphocytes produce cytokines, and when mature they become plasma cells and start Antib- odies production.  B lymphocytes require two different signals to become plasma cells that produce antibodies: Bind antigen with their surface bound antibody. Exposure to cytokines (interleukin 4 (IL-4) and IL-5) produced by T lymphocytes (T-helper cells), (figure 14). Fig. (14): Differentiation of B cells. 2- T lymphocytes, produced in bone marrow but mature in the thymus gland. They do not produce antibody molecules. T cells see antigen in a different way to B cells. They recognize peptide fragments of antigen complex with cell surface MHC (Major Histo- compatibility Complex). Types of T-cells: a- Cytotoxic T cells (CD8 positive), CD8+ cytotoxic T cells carry CD8 glycoprotein markers on their surface. They recognize antigen bound to MHC I class prote- in, mainly on virus-infected cells. They produce  cytotoxins which destroy cells infected with viruses and other intracellular organisms. The cytotoxic T cells activated under effects of two different signals:  Bind to antigen with their T-cell receptor.  Exposure to cytokine (interferon γ [IFN-γ] from T- helper cells called "Th1" T-helper cells. b- Helper T cells (CD4 positive), CD4+ helper T cells carry CD4 glycoprotein markers on their surface. They are important cells in humoral and a cellular immune response. They release cytokines, help to activate B lymphocytes, and modulate cellular immune responses. Helper T cells recognize antigen bound to MHC class II protein.  There are two basic types of T-helper cells: Th1 T-helper cells produce IFN-γ to activate the T- cytotoxic cells. Th2 T-helper cells produce IL-4 and IL-5 to activate B lymphocytes. Without T-helper cells, the patient’s adaptive im- mune systems (i.e., humoral and cellular immune responses) become much less effective at elimina- ting microbial invaders. c- Regulatory T cells (Treg), suppress the immune response in order to prevent over activity. These T cells may inhibit or destroy active lymphocytes. d- Memory T cells, remember an antigen and start a rapid response if that antigen is contacted again (figure 15 & 16). Fig.(15): of T cell activation and differentiation. Fig.(16): T cells types and functions. 3- Natural killer (NK) cells, are large granular lymph- oytes which are cytotoxic in the absence of prior stimulation. NK cell can kill virus infected cells, tumor or intracellular bacteria-infected cells with-out antigenic stimulation. NK cells represent a first line of defense to infecti-ons and tumor growth. (They produce cytokines and express receptors for immu- noglobulin), (figure 17 & 18). NK cell activation is determined by the balance of inhibitory and activating receptor stimulation, that means, if the inhibitory receptor signaling is more prominent, then NK cell activity will be inhibited, similarly, if the activating signal is dominant, then NK cell activation will result. Fig.(17): Natural killer cell. Fig.(18): Killing infected cell with NK cell. b- Granulocytes: 1- Neutrophils (polymorphonuclear cells (PMNs): are the cells with bactericidal activity that respond first to any type of infection or wound. Neutrophils are attracted by chemotactic factors to areas of inflammation. Those factors may be released by microbes (particularly bacteria) or may be genera- ted during complement activation as a consequence of an antigen–antibody reaction. Dead PMN and their debris become the primary component of pus (purulent), characteristic of many bacterial infections. 2-Eosinophils, play a role in the defense against parasitic infections. 3- Basophils, that play a major role in certain allergic responses. c- Monocytes: When monocytes migrate into tissues, the cell become large and the cellular metabolism increase. They differentiated to macrophages. The macroph- age have two important functions: phagocytosis and presentation of antigen to T cells to initiate the specific immune response. The mononuclear phag- ocytic system consists of monocytes circulating in the blood and macrophages in the tissues. Macrophage express surface receptors, including major histocompatibility y complex "MHC" class II molecules, and immunoglobulin "Fc" receptors on their surface, which aid phagocytosis and antigen presentation. d- Dendritic Cells, they originate from progenitor in the bone marrow through hematopoiesis, are named because they are covered with long membranous extensions that make them resemble nerve cell dendrites. Their main function is to phagocytose antigen and present it to helper T lymphocytes. They are classified according to their tissue location, Langerhans cells are found on skin and mucous membranes; interstitial dendritic cells populate the major organs such as the heart, lungs, liver, kidney, and the gastrointestinal tract; and dendritic cells are present in the T lymphocyte areas of secondary lymphoid tissue and the thymus. After capturing antigen in the tissue by phagocyto- sis or endocytosis, they migrate to the blood and to lymphoid organs, where they present antigen to T lymphocytes to initiate the acquired immune response. They are the most potent phagocytic cell in the tissue. Antigen presenting cells include (macrophages, dendritic cells, and B cells), (figure 19). Fig. (19): Antigen presentation process by Ag presenting cells.