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Download Unit 2.2.2 – Health and Disease Immunity
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SICM Tuition Unit 2.2.2 Health and disease Unit 2.2.2 – Health and Disease Health and Disease 1948 World Health Assembly: “Health is a state of complete physical, mental, and social well-being and not merely the absence of disease or infirmity.” Think about this for a moment. Many people associate health with not being physically ill. But in fact, healthcare professionals play a much wider role in ensuring people have a good quality of life. Disease: Very difficult to define. Here are some thoughts: “An abnormal condition of the body or mind that causes discomfort or dysfunction; distinct from injury insofar as the latter is usually instantaneously acquired.” “An alteration of a living body that impairs its functioning” “A condition of being sick from a particular cause” Immunity What is immunity? In order to remain healthy and able to function properly, the body must protect itself from disease-causing organisms, known as pathogens. The study of the mechanisms used to do this is known as immunology. The human body is well adapted to prevent pathogens (mainly bacteria, fungi and viruses) from entering in the first place. There are several ways in which this is achieved: - the skin forms a protective barrier against most pathogens o if the skin is cut, blood clotting helps to seal the wound - the mucus membranes of the nose and respiratory tract filter air using small hairs known as cilia and trap pathogens in the mucus for destruction by phagocytes. - anti-bacterial enzymes in saliva, sweat and tears help destroy pathogens - the stomach acid (pH ≈ 2) kills most pathogens, which may enter via food or water If, however, these mechanisms fail and pathogens do enter the body tissue or blood, an immune system is necessary to attack and destroy these pathogens and so prevent serious disease or even death. The immune system uses several specialised types of white blood cells: the most important being phagocytes and lymphocytes. SICM Tuition Unit 2.2.2 Health and disease Phagocytes Phagocytes are white blood cells, which can engulf and digest pathogens (as well as other foreign materials and dead or infected cells). They are non-specific in their actions and so will deal with any type of foreign material, which they come across. They have a distinctive appearance with a lobed nucleus and a grainy cytoplasm. Phagocytes are mainly found in the blood and the lymphatic system, especially in the lymph nodes. They are also capable of squeezing through tiny gaps between cells in the walls of capillaries and entering the tissue fluid, which surrounds every cell. granular cytoplasm lobed nucleus Phagocytosis – phagocytes engulfing/digesting bacteria phagocyte (nucleus not shown) bacterium giving out chemical lysosomes (containing powerful digestive enzymes) vacuole forming bacterium is trapped and some lysosomes move towards the vacuole and fuse with it. the bacterium is broken down and passes into the cytoplasm of the phagocyte Some definitions antigen - foreign protein (on the surface of the membrane of a virus or bacteria), which stimulates the production of a specific antibody antibody - chemical substance produced by a B-lymphocyte in response to exposure to a specific antigen. Antibodies They are typically made of basic structural units— each with two large heavy chains and two small light chains—to form, for example, monomers with one unit, dimers with two units or pentamers with five units. Although the general structure of all antibodies is very similar, a small region at the tip of the protein is extremely variable, allowing millions of antibodies with slightly different tip structures to exist. This region is known as the hypervariable region. Each of these variants can bind to a different target, known as an antigen. This huge diversity of antibodies allows the immune system to recognize an equally wide diversity of antigens. SICM Tuition Unit 2.2.2 Health and disease Lymphocytes Lymphocytes are white blood cells, which are involved in very specific immune responses against pathogens. They do not engulf and digest pathogens, but use other, more complicated processes to destroy them. Like phagocytes, lymphocytes circulate in the blood and lymph fluid and are also found within the lymph nodes. They have a large rounded nucleus, which almost fills the cell and a small amount of non-grainy cytoplasm. There are two main types of lymphocytes known as B-lymphocytes and TDifferent shaped antibodies lymphocytes. Both of these lymphocytes respond to the presence of specific types of foreign material in the body and bring about actions to remove these. Although they actually work in very different ways, the starting point for this is the recognition of the antigen. With regard to pathogens, antigens are surface protein molecules, which trigger an immune response. non-granular cytoplasm regularly-shaped nucleus The Immune response B and T-lymphocytes recognise specific pathogens that have invaded the body and set in motion mechanisms to destroy them: thus protecting the body from harm. This is known as the immune response. The function of B-lymphocytes B-lymphocytes are involved in the production of antibodies in response to antigens, which is called humoral immunity. On the surface of the membrane of B-lymphocytes are a number of specific antigen receptors. These are sites to which antigens on the surface of pathogens may become attached, leading to a sequence of events in which antibodies are produced to prevent the pathogens from causing harm. There are many thousands of specific types of B-lymphocytes and each is capable of recognising only one specific antigen from a pathogen. (For example, only one type of Blymphocyte will attach to the bacterium causing Tuberculosis, another one for Cholera) This specificity is due to the slight differences in the shape of the antigen receptor on each B-lymphocyte. SICM Tuition Unit 2.2.2 Health and disease Sequence of events - Pathogens enter the body. This may be via droplets in the air, by a vector (e.g. mosquito), via food, water or body fluids such as saliva, blood or semen. - Antigens on the surface of the pathogen come into contact with their specific antigen receptor on a B-lymphocyte. - The binding of the antigen to the antigen receptor activates the B-lymphocyte and causes it to divide producing a clone of identical B-lymphocytes. This is often known as a clonal explosion. - Most of the B-lymphocytes turn into plasma cells and the rest turn into memory cells. - The plasma cells begin to secrete antibodies against the pathogen concerned. Antibody molecules are secreted at a very high rate: up to 30 000 molecules per second. - The antibodies attach to the antigens on the pathogen and lead to the destruction of the pathogen. - Once the pathogens have been destroyed, the plasma cells eventually die and antibodies stop being secreted. But, the memory cells remain in the lymph nodes and the circulation in case of further infection with the same pathogen. The Immune response B-cell replicated many times: clonal explosion enlargement of B-cells to form plasma cells. These produce antibodies. pathogen with antigens antibodies B-cell is activated due to antigen memory B-cells: enable rapid response following subsequent exposure to the same antigen T-lymphocytes T-lymphocytes recognise foreign antigens and help in antibody production. Others bind to the pathogen and present the antigen to the B-lymphocytes.