Lec. 6 Cells and Organs of the Immune System PDF

Made with Xodo PDF Reader and Editor Lec.6 Cells and organs of the immune system ‫ سيف صالح‬.‫د‬  Made with Xodo PDF Reader and Editor Organs of the immune system: Organs concerned with immune reactions are called lymphoid organs. They contain...

Made with Xodo PDF Reader and Editor Lec.6 Cells and organs of the immune system ‫ سيف صالح‬.‫د‬  Made with Xodo PDF Reader and Editor Organs of the immune system: Organs concerned with immune reactions are called lymphoid organs. They contain lymphoid cells: Lymphoid organs are of 2 types: 1- Primary lymphoid organs 2- Secondary lymphoid organs 1-Primary lymphoid organs A- Thymus: is the site of T cell differentiation and maturation, cells found in thymus are: macrophages and dendritic cells. Organs of the immune system  Made with Xodo PDF Reader and Editor Bone marrow: site of B cell maturation in human, it is the site of generation of all circulating blood cells in the adult, including immature lymphocytes. 2-Secondary lymphoid organs: include: 1-Lymph nodes 2-Spleen 1- Lymph nodes: the organ in which immune response to antigens moved in lymph is initiated. It is the organ of the lymphatic system and the adaptive immune system. A large number of lymph nodes are linked throughout the body by the lymphatic vessels. They are major sites of lymphocytes that include B and T cells. Lymph nodes are important for the proper functioning of the immune system, acting as filters for foreign particles. 2- The spleen: is the major site of immune responses to antigens moved in blood. Produce some active substances, such as complement and has a role in filtration  Made with Xodo PDF Reader and Editor Cells of the immune system: 1-Lymphocytes: are white blood cells. There are two types of lymphocytes known as B lymphocytes and T lymphocytes, which are commonly referred to as B cells and T cells. Both types originate from stem cells in the bone marrow. From there, some cells travel to the thymus, where they become T cells. Others remain in the bone marrow, where they become B cells. A- T-lymphocyte cells: Mode of killing: 1-Direct by cell to cell action (cytotoxic cell). 2-Indirect by cytokines secretion (helper cell). T- lymphocyte  Made with Xodo PDF Reader and Editor There are several types of T cells: 1- Killer T cells: or cytotoxic, T cells scan the surface of cells in the body to see if they have become infected with virus or turned cancerous. If so, they kill these cells. 2- Helper T cells: Helper T cells “help” other cells in the immune system to start and control the immune response against foreign substances. 3- Regulatory T cells, or Tregs: Tregs control or suppress other cells in the immune system. They maintain tolerance to germs, prevent autoimmune diseases, and limit inflammatory diseases. 4- Memory T cells: Memory T cells protect the body against antigens that they have previously identified. They live long after an infection is over, helping the immune system remember previous infections.  Made with Xodo PDF Reader and Editor 2-B-lymphocytes Lymphocyte that matures in bone marrow and that responsible for humoral immunity. Mode of killing: - By specific immunoglobulin 3- Natural killer cells (NK cells) They form the third population of lymphocytes. They destroy the cancer cells and cells infected with virus, do not need antibody for activity, are activated by interferon and interleukin-2. Mode of killing: Kill by Ab dependent cell- mediated cytotoxicity (ADCC). B- lymphocyte  Made with Xodo PDF Reader and Editor 2- Macrophages: are type of white blood cell of the innate immune system that engulf and digest pathogens, such as cancer cells, microbes, cellular debris, and foreign substances. This process is called phagocytosis, which acts to defend the host against infection and injury. 3-Neutrophils: are type of phagocytic white blood cell and part of innate immunity. They make up 40% to 70% of all white blood cells in humans. Neutrophils are one of the first responders of inflammatory cells to migrate toward the site of inflammation. Neutrophils are moved to the site of injury within minutes following trauma and are the hallmark of acute inflammation. Made with Xodo PDF Reader and Editor Macrophage Neutrophil  Made with Xodo PDF Reader and Editor The complement system: It is a part of the humoral, enhances the ability of antibodies and phagocytic cells to clear microbes and damaged cells from an organism, promote inflammation, and attack the pathogen's cell membrane. Despite being part of the innate immune system, the complement system can be brought into action by antibodies generated by the adaptive immune system. The complement system consists of a number of small, inactive, liver synthesized protein precursors circulating in the blood. When stimulated by one of several triggers, proteases in the system cleave specific proteins to release cytokines and initiate the complement system. The end result of this complement activation is stimulation of phagocytes to clear foreign and damaged material, inflammation to attract additional phagocytes, and activation of the cell-killing membrane attack complex. Complement Activation: There are three separate pathways which activate the complement system:  Made with Xodo PDF Reader and Editor 1- Classical pathway: activated by antibody-antigen complexes (immune complexes) on pathogen surfaces. 2- Mannose-binding lectin pathway: activated when mannose-binding lectin binds to the carbohydrate molecule mannose on pathogen surfaces. 3- Alternative pathway: C3 reacts directly with pathogen surfaces. All three of these pathways act to generate the enzyme C3 convertase.his cleaves C3 into two parts (C3a and C3b) and activates the rest of the cascade Complement  Made with Xodo PDF Reader and Editor Complement Functions of the complement: 1- Cell lysis: The membrane attack complex can lyse a broad spectrum of cells, such as: G-ve bacteria, parasite viruses and tumor cells. 2- Inflammatory response: C3a, C4a and C5a (called anaphylatoxin) bind to complement receptors on mast cell and basophiles. This induce degranulation with release of histamin. 3- Opsonization: C3b is the major opsonin of the complement system. 4- Clearance of the immune complexes.  Made with Xodo PDF Reader and Editor Membrane attack complex Functions of the complement system  Made with Xodo PDF Reader and Editor Major histocompatibility complex (MHC): The major histocompatibility complex can be defined as a tightly linked cluster of genes whose products play an important role in intercellular recognition and in discrimination between self and non-self. These genes code for antigens which involve the determination of the compatibility of the transplanted tissue. The compatible tissues will be accepted by the immune system while the histo-incompatible ones are rejected. The rejection of foreign tissue leads to an immune response to cell surface molecules The Major Histocompatibility complex is a genetic locus that encodes the glycoprotein molecules (transplantation antigens) which are responsible for tissue rejection of grafts between genetically unidentical individuals. It is also the molecule that binds the peptide antigens processed by Antigen-presenting Cells and presents them to T- cells, hence they are responsible for antigen recognition by the T-cell receptors. Unlike the B-cell receptors that directly interact with the antigens, the T-cell receptors have an intertwined relationship with the MHC molecule, in that T-cell  Made with Xodo PDF Reader and Editor receptors can only receive and bind processed antigens in form of peptides that are bound to the MHC molecule, and therefore, T-cell receptors are specific for MHC molecules. In humans, the Major Histocompatibility complex is known as Human Leukocyte Antigen (HLA). There are three common MHC molecules i.e class I, class II, and class III MHC proteins The function of T-cells on interaction with the MHC molecules reveals that the peptide antigens associated with class I MHC molecules are recognized by CD8+ cytotoxic T-lymphocytes (Tc cells) and MHC class-II associated with peptide antigens that are recognized by CD4+ Helper T-cells (Th cells). Gene Products of HLA complex: Class I MHC genes encode glycoproteins expressed on the surface of nearly all nucleated cells; the major function of the class I gene products is presentation of endogenous peptide antigens to CD8+ T cells. Class II MHC genes encode glycoproteins expressed predominantly on APCs (macrophages, dendritic cells,  Made with Xodo PDF Reader and Editor and B cells), where they primarily present exogenous antigenic peptides to CD4+ T cells. Class III MHC genes encode several different proteins, some with immune functions, including components of the complement system and molecules involved in inflammation. An antigen-presenting cell (APC): is a cell that displays an antigen bound by major histocompatibility complex (MHC) proteins on its surface; this process is known as antigen presentation. T cells may recognize these complexes using their T  Made with Xodo PDF Reader and Editor cell receptors (TCRs). APCs process antigens and present them to T cells The HLA molecules: 1- HLA-class I molecule: Class I molecules react with CD8 T cells. These lymphocytes often have a cytotoxic function, requiring them to be capable of recognizing any infected cell. 2- HLA-class II molecule: Class II molecules react with CD4-T cells, have a helper function Functions of HLA: 1- Role in recognition of antigens 2- Role in cell-cell interaction in immune system Clinical significance of HLA: 1- Organ transplantation 2- Transfusion therapy 3- Disputed paternity 4- Anthropological studies  Made with Xodo PDF Reader and Editor The role of the complement system and HLA in oral diseases: Components of the complement system are present in the periodontal tissue and the system is activated in periodontitis. Continuous complement activation and modulation by bacteria within the biofilm in periodontal pockets, however, may enhance local tissue destruction, providing the biofilm with both essential nutrients and space to grow. The associations have been documented between HLA antigens and periodontitis