another question
can anyone give me a quick summary about all that we have to know about immunity in 3/4 biol? want to swift through the irrelevant things and my textbook is rather unclear. Also, can someone please summarise the first, second and third line of defence? thanks 
Immunity has three lines of defence, the first line, second line and third line of defence. The first and second are non-specific, also known as innate, whereas the third line of defence is specific, also known as adaptive immunity. The first line of defence is comprised of chemical and physical barriers that non-specifically prevent any foreign particle/cell/substance to enter the organism/human body. Some chemical examples include the mucus that is sticky and traps foreign substances, lysozyme in tears that destroy bacteria near your eyes and on your face, stomach acid that acidifies the pathogens and even intact skin which constantly acts as a barrier against pathogens. Once the first line is breached, and a pathogen has successfully entered the body, the second line of defence comes to play. There are chemical/cellular components and processes that compose this line of defence. Processes include fever, in which macrophages release interleukin to stimulate the hypothalamus (the thermostat of the body) to increase the core bodily temperature, which will disfunction any pathogenic metabolisms, there is phagocytosis, the process whereby pathogens are engulfed by phagocytes, and there is inflammation, where mast cells granulate to release histamine, the chemical that causes vasodilation and increases the permeability of capillaries so that blood flow can rush to the infected site, and in this blood there will be an influx of white blood cells that will fight off the infection. Some chemicals/cells utilised in the second line of defence include complement proteins, which will bind to the outer membrane/cell wall/capsule of bacterial cells and essentially agglutinate the bacteria, and attract phagocytes to engulf the complex, also there are interferons, proteins secreted by virally-infected cells to stimulate near-by cells to transcribe enzymes for defence against this virus, in the case that it successfully reproduces in its host cell and continues its invasion in near-by cells. Some cells of the second line defence include phagocytic cells, including macrophages, dendritic cells and neutrophils, all of which engulf pathogens and destroy them. In the third line of defence, there is specificity. This means that each type of pathogen will induce a specific immune response. For example, if you got infected with measles, your body would respond differently to it than if you were infected with a flu. So, you have these wonderful cells known as lymphocytes. They are called LYMPHocytes because they are primarily found in the lymph nodes all around your body. There are two main classes of lymphocytes, B cells and T cells. The B cells are the humoral cells, and the T cells are the cell-mediated ones. Now, in order to initiate the third line of defence, the second line must be breached. So, a phagocyte form the second line of defence must have already been in contact with a pathogen. It will engulf this pathogen via phagocytosis and encapsulate it in a phagosome (vesicle containing the pathogen). This phagosome will then fuse with a lysosome and the digestive enzymes will degrade this pathogen. The pathogenic/antigenic fragments will then be placed on cytoplasmic MHC II markers inside the cell, and then they will fuse with the cell membrane, and become transmembrane MHC II markers, containing specific antigenic fragments. This phagocyte will encounter a T-helper cell located in the lymph tissue, so it must travel through the lymph stream to the lymph nodes and start its journey to find the specific T-helper cell that has the exact complementary T-cell receptor for that specific antigen. It will continue the search until it finds its match, once found, the T-helper cell will become activated and proliferate into many more T-helper cells specific for that antigen. The T-helper cell will then be responsible to 'help' or activate the cytotoxic T-cells and activate the B-cells. The activated helper T-cells will bind with cytotoxic T cells complementary to this antigen and activate them, and these will patrol the body and destroy any infected cell (mostly the case of viruses as they need host cells to reproduce). This T-helper cell will also locate a specific B-cell that has pre-engulfed the same pathogen prior, and has displayed its antigenic fragments on its own MHC II markers, and the helper cell will bind with the B-cell and activate it, stimulating proliferation into two types of B-cells, plasma B-cells and memory B-cells. The plasma B-cells will be infused with rough endoplasmic reticulum so that they can mass produce and secrete specific antibodies to agglutinate the pathogens, and the memory B-cells will be stored in the lymph nodes and hence the next time the same pathogen invades, these memory B-cells will be abundant and ready to detect this pathogen, to repeat the same process of proliferation.
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