Chapter 2 Organs and Cells of Immune System PDF

Chapter Two : Organs and Cells of Immune System  Learning Objectives Upon completion of this lesson the student will be able to: Describe cells and organs of immune system Describe lymphoid tissue by primary or secondary, locations of specialized tissues, cells produced and...

Chapter Two : Organs and Cells of Immune System  Learning Objectives Upon completion of this lesson the student will be able to: Describe cells and organs of immune system Describe lymphoid tissue by primary or secondary, locations of specialized tissues, cells produced and key role in immunity. Describe the morphology, source and role of macrophages, natural killer cells, cytotoxic, helper, suppressor or B lymphocytes and plasma cells. Discuss the role of surface markers in cells involved with immunity, referring to specific markers used to differentiate T and B lymphocytes.  Outline 2.1. Organs of the immune system 2.2. Cells of the immune system  The immune sysstem has different The yellow area give examples of how the natural body defense systems could be compromised or how the pathogens set an environment for infection  Definition of Pathogenicity and Virulence Pathogenicity: is the capacity to produce disease in the host. Pathogenicity depends on the ability of an organism to invade and avoid the hosts defense mechanisms, and to multiply. Virulence: refers to the intensity of the disease produced by pathogens and it varies among different microbial species. The virulence of a pathogen can be decreased by attenuation, the weakening of the disease producing ability of the pathogen. Attenuation can be achieved by repeated sub culturing on laboratory media Pathogen: microorganisms capable of causing disease.  True pathogens: capable of causing disease in healthy persons with normal immune defenses. E.g. Influenza virus, malarial protozoan.  Opportunistic pathogens: cause disease when the host’s defenses are compromised or when they grow in part of the body that is not natural to them. E.g. Pseudomonas spp & Candida albicans Immunogenicity : the ability to induce an immune response in the host i.e. the degree to which an antigen elicits an immune response  2.1. Organs and Cells of the immune system Introduction The immune system consists of many different organs and tissues that are found throughout the body. These organs can be classified functionally into two main groups. The primary lymphoid organs provide appropriate microenvironments for the development and maturation of lymphocytes. This includes Thymus, Bone marrow, Fetal liver Primary Lymphoid Organs: the primary site of lymphocyte development (lymphopoiesis) are known as Primary lymphoid organs (PLO). Lymphocytes develop from lymphoid stem cells, multiply, and mature into useful cells known as immuno-competent cells. the PLO includes the bone marrow and thymus.  Thymus: it is a tiny organ located in the upper chest which helps to mature a particular type of white blood cells. The thymus provides an inductive environment for forming T lymphocytes from hematopoietic stem cells. The lymphocyte fully mature and differentiate into effective T lymphocyte in the thymus  Bone Marrow: red cells, plasma cells, several type of white blood cells and other immune cells are produced from stem cells found in the spongy interior of the bones. Every day, the bone marrow produces billions of new blood cells and release them into the blood.  The primary site of B- cell maturation and propagation in adults is the bone marrow, where all other circulating blood cells are produced. The process of producing all blood cells through out fetal development is known as hematopoiesis. It occurs in the blood island of the yolk sac followed by the liver and the spleen. Secondary Lymphoid Organs In addition to the primary lymphoid organs there are few other lymphoid organs known as secondary lymphoid organ(SLO). The spleen and lymph nodes are the two most significant and well organized SLO The SLO sustain and initiate an adaptive immune response Antigen induced lymphocyte activation takes place in the SLO. Until the come in contact with their particular antigen mature lymphocyte travel back and forth b/n the blood and SLO i.e. clonal growth and affinity maturation are by activation Lymph node: immune cells found in lymph nodes examine foreign pathogens introduced into the body. These tiny glands filter and kill them to prevent the pathogens from spreading to other body areas. They are a components of the lymphatic system in our body. Here the individual lymphocytes are activated , replicated and sent to combat that specific invader Numerous lymph node can be found through out the body, especially in the groin, armpits and neck Lymph nodes that are swollen and painful are a sign that the body is fighting infections Spleen: it is the SLO that is located in the left abdominal area. Spleens are designed to filter blood, captured blood borne antigens and react to systemic infections  The primary function of the spleen are:  Production of immune cells to fight pathogens or their antigen  Removal of particulate matter and aged blood cells (RBCs)  Production of blood cells during foetal life The spleen produces antibodies in its white pulp and eliminates antibody coated blood cells and germs through lymph nodes and blood circulation.  2.1. Organs of the immune system Tertiary lymphoid tissues which normally contain fewer lymphoid cells than secondary lymphoid organs, Can import lymphoid cells during an inflammatory response. Most prominent of these are cutaneous-associated lymphoid Tissues (CALT). Once mature lymphocytes have been generated in the primary lymphoid organs, they circulate in the blood and lymphatic system, a network of vessels that collect fluid that has escaped into the tissues from capillaries of the circulatory system and ultimately return it to the blood  Cells of the Immune System The cells of the immune system includes: A. Lymphocytes: are small round cells found in peripheral blood, lymph, lymph nodes, lymphoid organ and in tissues It represents 20-45% of total cells in peripheral blood and 99%of total cells in lymph and lymph node Broadly lymphocytes are divided into three sub populations namely T lymphocyte, B lymphocyte and Natural killer cell B. Phagocytic cells: monocytes and macrophages are mononuclear phagocytic cells Granulocyte –monocyte progenitor cells differentiate into promonocytes and neutrophil Promonocyte leaves the bone marrow and enters into bloodstream where they differentiate into mature monocytes Monocyte circulates in blood for about 8 hours, during which they enlarges and then enter into tissues and differentiates into specific macrophages and dendritic cells  Monocytes: it is a single lobed with kidney shaped nucleus with a size of 12-15µm. Monocyte accounts for 2-8% of blood leukocyte. Immunological function of monocyte  Helps in antigen processing and presentation  Release cytokines Macrophages: monocytes migrates to tissue and differentiates into macrophages Differentiation of monocyte into macrophages involves the following changes  Cell enlarges 5-10 folds  Intracellular granules increases in number and complexity  Increase phagocytic ability  Produces higher level of hydrolytic enzymes and cytokines Immunologic function of macrophages  Phagocytosis  Antigen presenting to T cell  Secretion of granulocyte monocyte colony stimulating factors  secretion of lymphokines to activate inflammatory response C. Granulocytic Cells Granulocytic cell includes neutrophil, eosinophil and basophils  Neutrophil: it is 11-14 µm in diameter with multilobed nucleus with granules in cytoplasm It constitutes 50-70% of total circulating leukocyte Immunological function of neutrophil  Phagocytic role in acute inflammatory response  It is the first immune cell to respond in inflammation  Eosinophil: it is11-15µm in diameter with heavily granulated bilobed nucleus Immunological functions of eosinophil  Granules contain various hydrolytic enzymes that kill parasites which are too large to phagocytized by neutrophil  Provide allergic inflammation Basophils: are non phagocytic cells found in small numbers in blood and tissue Cytoplasm contains large number of prominent basophilic granules containing histamine, heparin, serotonin and other hydrolytic enzymes. The immunological function of basophil is providing anaphylactic allergic reaction D. Dendritic cells: they have long cytoplasmic extensions that resembles to dendrites of nerve cells They have highly pleomorphic with a small central body and many long needle like processes They are antigen presenting cells Immunological functions of dendritic cells  Involved in antigen presenting to T cells during primary immune response  They do have little role in phagocytosis  2.3. Innate and Adaptive Immune Response The immune system is most likely developed to reduce the harmful impact of infectious agents on the host This defense approach is based on the coordinated activities of innate and adaptive immune system components. This coordinated approach detects and target infectious agents for containment, killing or expulsion by the body’s defense mechanisms. These immunological process are responsible for decreasing the pathogen burden of an infected host to maintain homeostasis that is considered to be infection resistance. Immune resistance to infection helps to minimize the amount of dysfunction imposed on the host tissue during infection. The immune defense system consists of immune cells and products.  The immune system fights germs and foreign substances on the skin in the tissue of the body and in bodily fluids such as blood. The innate immune system: fast and general effectiveness It is the body’s first line of defense against germs entering the body. It responds in the same way to all pathogens and foreign substances, which is why some times reffered as nonspecific immune system. It acts very quickly. The innate immune system consists of  Protection offered by the skin and mucous membranes  Protection offered by immune system cells and proteins(enzymes) All the outer and inner surfaces of the human body are the key parts of the innate immune system The closed surface of the skin and all mucous membranes already forms a physical barrier against germs, which protects them from entering. Additionally, chemical substances like acid, enzymes or mucus prevent pathogens from gaining foothold.  Movements created , fore example, by hair like structures in bronchi (cillia) or bowl muscle stop germs from settling in the body. Tear fluid, sweats and urine have the same effects. If these pathogen pass the skin and mucous membrane and enter the body innate immune system activates special immune system cells and enzymes. These immune cells move to site of infection and release substances that make the blood vessel wider and more permeable. This causes the area around the infection to swell, heat up and redden and inflammation results. A fever may also develop. The blood vessel expands further and more immune system cells like phagocytic cells, natural killer cells and other cell products (proteins) will arrive. Bacteria or virus that enter the body can be stopped right away by scavenger cells(phagocytes). These cells enclose and digest the pathogen. The remains of these germs move to the surface of the phagocytic cells to be detected by the adaptive immune system  There are also other type of immune system cells that release substances to kill bacteria and various germs. Both the germs and immune system cells will die and decay during an immune response and forms pus. Several proteins (enzymes) help the cells of innate immune system. The tasks of these proteins(enzymes) include:  Marking germs as targets for phagocytic cells  Attracting other immune cells from the bloodstream  Destroying bacteria cell walls to kill them  Fighting viruses by destroying viral envelope or cells that have been infected with viruses The other immune cells involved in innate immune system are the natural killer cells  They specialize in identifying cells infected by viruses and/or cells that are tumorous  The adaptive immune system: the adaptive immune system takes over if the innate immune system fails to destroy the pathogens. It specifically targets the type of pathogen that is causing the infection. To do this first it needs to identify the pathogen i.e. it is slower to respond than the innate immunity but when it does it is more accurate Being it ha the potential to remember the pathogen, so the next time a known causative agent is encountered, the adaptive immune system can respond faster. This is why there are some illness you can only get once in your life and you can not even noticed the second infection or it is at least milder The adaptive immune system is made up of:  T lymphocytes  B lymphocytes  Antibodies in blood and other body fluids  T lymphocytes (T cells) are produced in bone marrow and then move to the thymus through the bloodstream, where they mature. T cells have three main jobs  It serves as chemical messengers to activate other immune system cells in order to start the adaptive immune system( T helper cells)  They detect cells infected by viruses or tumorous cells and destroy them (cytotoxic T cells)  Some T helper cells become memory T cells after infection has defeated. It can remember the defeated pathogen and induces faster adaptive immune response T cells have detection features on their surface that can attach to germs like a lock that one particular key will fit. i.e. If a pathogen attaches to a matching T cells the T cell starts to multiply; creating more T cells specialized to that pathogen. The cell produced in the bone marrow, the cell that matures will be specifically against the that pathogen…immunity customzed  B lymphocytes(B cells) are made in the bone marrow and then mature thereto become specialized immune system cells The B cells are activated by T helper cells. T helper cells contact B cells that match the same germs that they do. This activates the B cells to multiply and to transform themselves to plasma cells. These plasma cells quickly produce very large amount of antibodies and release them into the bloodstream Because only the B cells that match the attacking pathogen are activated, only the exact antibodies that are needed will be produced Some activated B cells will be transformed into memory and become part of the memory adaptive immune system.  Summary The cells that participate in the immune response are white blood cells. The lymphocyte is the only cell to possess the immunologic attributes of specificity, diversity, memory, and self/non self recognition The primary lymphoid organs provide sites where lymphocytes mature and become antigenically committed Secondary lymphoid organs capture antigens and provide sites where lymphocytes become activated by interaction with antigens.  Functions of the immune system To protect from pathogens – Intracellular (e.g. viruses and some bacteria and parasites) – Extracellular (e.g. most bacteria, fungi and parasites) To eliminate modified or altered self  Effects of the immune system Beneficial: Protection from Invaders Elimination of Altered Self Detrimental: Discomfort and collateral damage (inflammation) Damage to self (hypersensitivity or autoimmunity) Development of immune Cells Polymorphonuclear Neutrophilic Leukocytes, a.k.a., PMNs. They are shorter lived than macrophages but have greater killing power. These are Cells of the adaptive immune system