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Immunology Lecture 1 WEEK 1 ACADEMIC YEAR 2015-2016 INTRODUCTION AND OVERVIEW What Is Immunology? Immunology is the science that studies the immune system. WHAT THE IMMUNE SYSTEM DOES? The major functions of the immune system are: 1 – Protection of the body against damage caused by infectious agents or any other foreign substance in its environment. 2 – Removal of damaged tissues or cells (Homeostasis). 3 – Check for development of unwanted cells (altered or modified cells) within the body and coordinate its removal (Surveillance) WHAT DOES IMMUNOLOGY AIM FOR? To give answers the following questions: 1 – How does the body defend itself from pathogens? 2 – How is a pathogen that succeeded in entering the body, eliminated? 3 – How does the body remember a prior exposure to a pathogen then responds faster and more efficiently when the same pathogen is again encountered? 4 – How to make use of all these studies: To develop vaccines and immune-therapies. AGAINST WHAT THE IMMUNE SYSTEM IS ACTIVE? Against foreign substances e.g. microbes, while sparing the body’s own tissues. This means: 1 - Ability to detect (recognize) the foreign substance 2 - Ability to remove (eliminate) this foreign substance. Important definitions The reaction against a microbe or any other foreign substance is called immune response. The lack of reactivity to self components is called self tolerance. Any substance that can be recognized by the immune system and induce an immune response is called antigen or immunogen. Components of our own tissues can be immunogenic in other hosts so they are called self antigens. WHAT IS THE STRUCTURE OF THE IMMUNE SYSTEM? A complex network of: Cells Tissues & Soluble Molecules. Cells of the immune system The cells are widely distributed throughout the body but occur predominantly in lymphoproliferative tissues e.g. bone marrow, thymus, spleen, lymph nodes. All immune cells arise from a stem cell in the bone marrow. They are mainly leukocytes. All immune cells have the ability to recognize and eliminate injurious substances Molecules A wide variety of molecules are involved e.g. Antibodies Cytokines Acute phase proteins Complement Adhesion molecules Functionally, the immune system is divided into 2 complementary units: 1 – The innate immune system: the 1st to interact with invaders, and it is fast. 2 – The adaptive immune system: deals with invaders that escape destruction by innate mechanisms. HOW DOES THE IMMUNE SYSTEM DISTINGUISH BETWEEN SELF ANTIGENS AND FOREIGN ANTIGEM]NS? HOW IT DOES DISTINGUISH BETWEEN DIFFERENT ANTIGENS? The immune system can recognize chemical differences between macromolecules (molecular recognition). This very important property is achieved through receptors on the surface of immune cells. HOW DOES THE IMMUNE SYSTEM ELIMINATE PATHOGENS? Through different effector mechanisms that handle a wide range of organisms causing different types of infections. These mechanisms differ according to the location (lifestyle) of the organism: intracellular or extracellular. EXAMPLES OF EFFECTOR MECHANISMS 1 – Neutralisation by specific antibodies 2 – Phagocytosis by phagocytic cells 3 – Use of complement system 4 – Cytotoxic reactions 5 - Inflammation PROTECTION IS AT SEVERAL LEVELS 1 – External defenses (barriers to infection) 2 – Innate immunity 3 – Adaptive immunity Always remember: Infection does not necessarily mean disease. Why? Because the immune system in most cases is able to eliminate infection before disease occurs. Disease occurs only when: 1 – The infectious dose is so high 2 – The virulence of the organism is great 3 – Immunity is compromised Always remember: For the most part, the immune system has beneficial effects. However, there can be some bad effects as well, Example: During inflammation, there may be local discomfort and some damage to healthy tissues. WHAT HAPPENS IF THE IMMUNE SYSTEM MALFUNCTIONS? When immune mechanisms fail Immunopathology. The immune system may fail in one of 3 ways: 1 – When any of its components is defective, this is called immunodeficiency . 2 – When the immune system reacts against self antigens, this is called autoimmunity. 3 – When an exaggerated response occurs in certain individuals to harmless environmental antigens e.g. pollens, this is called Allergy or hypersensitivity reactions. Other situations: when the immune system acts normally but responses are inconvenient In graft rejection