immunity aqa a level bio notes

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in defending against pathogens: ● White blood cells known as phagocytes have surface proteins that act as receptors and bind to the proteins (antigens) on the surface of pathogens This enables pathogens to be engulfed and digested The antigens that were found on the pathogen can then be presented on the surface of the phagocyte (now an antigen-presenting cell) This is then used to recruit other cells of the immune system, leading to a specific immune response This document is available free of charge on StuDocu.com Downloaded by Ogheneyoma Adjekpovu ([email protected]) Antigens Every microorganism has markers that identify it called antigens (macromolecules). Antigens allow cell-to-cell recognition Antigens are found on cell surface membranes, bacterial cell walls, or the surfaces of viruses: O ● Some glycolipids and glycoproteins on the outer surface of cell surface membranes act as antigens o Antigens can be either self-antigens or non-self antigens: ● o Self-antigens: o O Non-self antigens: Antigens not produced by the organism's own body cells e.g., the antigens found on pathogenic bacteria/viruses or if a person receives a different blood type during a transfusion O Immune system recognises these antigens as foreign o Non-self antigens stimulate an immune response Antigen Variability Antigens produced by the organism's own body cells Self-antigens do not stimulate an immune response Some pathogens exhibit antigen variability O The antigens present on their surface change frequently due to genetic O mutations This poses a problem for the immune system of many mammal hosts as lymphocytes and memory cells produce a specific immune response The surface receptors on lymphocytes and memory cells are complementary in shape to only one antigen When the antigen on a pathogen changes the lymphocytes and memory cells can no longer bind O As a result, there is no secondary immune response o The host gets infected and suffers from the disease again The cold virus and flu virus are common pathogens that exhibit antigen variability O Individuals can catch the cold and flu year after year as the antigens on the viruses change and are not recognised by their immune system O Downloaded by Ogheneyoma Adjekpovu ([email protected]) Phagocytosis ● ● ● Phagocytes: White blood cells that are produced continuously in the bone marrow Stored in the bone marrow before being distributed around the body in the blood Responsible for removing dead cells and invasive microorganisms Have a multi lobed nucleus Carry out a non-specific immune response Phagocytosis: process or recognising and engulfing a pathogen Chemical products of pathogens or dead, damaged and abnormal cells act as ATTRACTANTS. Cause phagocytes to move towards the pathogen along a concentration gradient. (chemotaxis) Phagocytes have receptors on their cell surface membrane that recognise and attach to chemicals on the surface of the pathogen. Phagocytes engulf the pathogen to form a vesicle called a phagosome. Lysosomes move towards the phagosome and fuse with it. Lysozymes are present in the lysosome. Lysozymes destroy ingested bacteria by hydrolysis of their cell walls. Soluble products from the breakdown of the pathogen are absorbed into the cytoplasm of the phagocyte. This document is available free of charge on StuDocu.com Downloaded by Ogheneyoma Adjekpovu ([email protected]) The T Lymphocyte Response Lymphocytes: type of white blood cell ● ● ● Important role in specific immune response Smaller than phagocytes ● Large nucleus that fills most of the cell Produced in the bone marrow before birth T-lymphocytes and the cellular immune response Two types of lymphocytes: T-lymphocytes (T cells) B-lymphocytes (B cells) cell mediated response Immature T-lymphocytes leave the bone marrow to mature in the thymus Mature T-lymphocytes have specific cell surface receptors called T cell receptors These receptors have a similar structure to antibodies and are each specific to one antigen. T-lymphocytes are activated when they encounter (and bind to) their specific antigen that is being presented by one of the host's cells (host cells being the human's own cells). This antigen-presenting host cell might be a macrophage or a body cell that has been invaded by a pathogen and is displaying the antigen on its cell surface membrane. These activated T-lymphocytes (those that have receptors specific to the antigen) divide by mitosis to increase in number. Forms clones of genetically identical cells These cloned T-lymphocytes differentiate into: O T-helper cells Releases chemicals to stimulate more phagocytosis Activates B Cells: (link to humoral response) Cytotoxic T cells (killer T cells) Uses perforin to make holes in the cell membrane of infected cells to kill them. O T-Memory Cells Remain in blood for a faster secondary immune response (If person is re-infected) O Cloned T-lymphocytes also: O Stimulate phagocytes to engulf pathogens by phagocytosis O Stimulate B-cells to divide and secrete antibody Downloaded by Ogheneyoma Adjekpovu ([email protected]) The Role of Helper T cells Helper T cells assist other white blood cells in the immune response They release cytokines (hormone-like signals) which stimulate: O The maturation of B-lymphocytes into antibody-secreting plasma cells O The production of memory B cells O The activation of cytotoxic T cells, which destroy virus infected cells and tumour cells An increased rate of phagocytosis O This document is available free of charge on StuDocu.com Downloaded by Ogheneyoma Adjekpovu ([email protected]) The B lymphocyte Response ● ● ● ● ● ● ● B-lymphocytes (B cells) remain in the bone marrow until they are mature and then spread through the body, concentrating in lymph nodes and the spleen. Once mature, each type of B-lymphocyte cell can make one type of antibody molecule At this stage, the antibody molecules do not leave the B-lymphocyte cell but remain in the cell surface membrane. ● humoral immunity Part of each antibody molecule forms a glycoprotein receptor that can combine specifically with one type of antigen (complimentary antigen). If that antigen enters the body, B-lymphocyte cells with the correct cell surface receptors will be able to recognise it and bind to it. Antigen enters the B cell by endocytosis and is presented on its surface. T helper cells bind to the antigens on surface Stimulates B cell to divide by mitosis Forms clones of identical B cells. CLONAL SELECTION PLASMA CELLS Secrete lots of antibodies (specific to the antigen) into the blood plasma, lymph or linings of the lungs and the gut. These plasma cells are short-lived but the antibodies they have secreted stay in the blood for a longer time. The other B-lymphocytes become memory cells that remain circulating in the blood for a long time This response to a newly encountered pathogen is relatively slow MEMORY CELLS When B-lymphocytes divide repeatedly by mitosis (clonal expansion), they differentiate into two main types of cell: Plasma cells Memory cells primary immune response Secondary immune response If the same antigen is found in the body a second time, the memory cells recognise the antigen, divide very quickly and differentiate into plasma cells (to produce antibodies) and more memory cells. This response is very quick, meaning that the infection can be destroyed and removed before the pathogen population increases too much and symptoms of the disease develop. This response to a previously encountered pathogen is, relative to the primary immune response, extremely fast. Provides long-term immunity. Memory cells themselves cannot produce antibodies. Downloaded by Ogheneyoma Adjekpovu ([email protected]) invading pathogen 2 3 antigen B cell processed antigen B cell B-cell antibody activated helper T cells cloning by mitosis plasma cell antibody secreted memory cell ▲ Figure 2 Summary of role of B cells in humoral immunity circulate in blood and tissue fluid in readiness to respond to a future infection by the same pathogen by dividing and developing into plasma cells that produce antibodies. (-secondary response) attach to antigens on pathogen and 1 The surface antigens of an invading pathogen are taken up by a B cell. The B cell processes the antigens and presents them on its surface. Helper T cells (activated in the process described in Topic 5.3) attach to the processed antigens on the B cell thereby activating the B cell. The B cell is now activated to divide by mitosis to give a clone of plasma cells. 4 5 The cloned plasma cells produce and secrete the specific antibody that exactly fits the antigen on the pathogen's surface. destroy it (-primary response) 6 The antibody attaches to antigens on the pathogen and destroys them (see Topic 5.5). 7 Some B cells develop into memory cells. These can respond to future infections by the same pathogen by dividing rapidly and developing into plasma cells that produce antibodies. This is the secondary immune response. This document is available free of charge on StuDocu.com Downloaded by Ogheneyoma Adjekpovu ([email protected]) Antibodies: Structure & Functions Globular glycoproteins called immunoglobulins ANTIGEN-BINDING SITE - MATCHES THE EPITOPES OF ANTIGENS VARY BETWEEN ANTIBODIES AS THEY HAVE SPECIFIC AMINO ACID SEQUENCES UNIQUE TO AN ANTIGEN VARIABLE REGION Quaternary structure 2 Heavy (long) polypeptide chains 2 Light (short) polypeptide chains Bonded by disulphide bridges ● DISULFIDE BRIDGES (-S-S-) CONSTANT REGION ANTIGEN EPITOPE THE PART OF THE ANTIGEN WHICH BINDS TO THE ANTIBODY LIGHT POLYPEPTIDE CHAIN HEAVY POLYPEPTIDE [CHAINS HINGE REGION GIVES FLEXIBILITY WHICH ALLOWS THE ANTIGEN-BINDING SITE TO BE PLACED AT DIFFERENT ANGLES WHEN BINDING TO ANTIGENS (IT IS NOT PRESENT ON ALL IMMUNOGLOBULINS) Each polypeptide chain has: Constant region Variable region Constant region: determines mechanism used to destroy the antigens Variable region: amino acid sequence in this region are different for each antibody. where the antibody attaches to the antigen to form an antigen-antibody complex Antigen-binding site: generally composed of 110 to 130 amino acids and includes both the ends of the light and heavy chains. The antigen-binding sites vary greatly: gives the antibody its specificity for binding to specific antigens. The sites are specific to the epitope (the part of the antigen that binds to the antibody) Hinge Region: gives flexibility to antibody molecule. Allows the antigen-binding site to be placed at different angles when binding to antigens Downloaded by Ogheneyoma Adjekpovu ([email protected]) Function ● Antibodies are produced by B-lymphocytes Antibodies bind to specific antigens that trigger the specific immune response. Every antigen has one antibody Antibodies are divided into five major classes (isotypes), each with a different role: Antibodies can combine with viruses and toxins of pathogens (e.g. bacteria) to block them from entering or damaging cells Antibodies can act as anti-toxins by binding to toxins produced by pathogens which neutralises them making them harmless. Antibodies can attach to bacteria making them readily identifiable to phagocytes, this is called opsonisation. Once identified, the phagocyte has receptor proteins for the heavy polypeptide chains of the antibodies, which enables phagocytosis to occur. Antibodies can attach to the flagella of bacteria making them less active, which makes it easier for phagocytes to perform phagocytosis. Antibodies act as agglutinins causing pathogens to clump together (agglutination). This is possible because each antibody has two antigen binding sites. This forms clumps of pathogens which makes it easier for phagocytes to locate them. The antibodies then act as markers to stimulate phagocytes to engulf the pathogens. Antibodies (together with other molecules) can create holes in the cell walls of pathogens causing them to burst (lysis) when water is absorbed by osmosis. This document is available free of charge on StuDocu.com Downloaded by Ogheneyoma Adjekpovu ([email protected]) Antigen-Antibody Complex ● ● VARIABLE REGIONS ● ● ● An antigen and its complementary antibody have complementary molecular shapes This means that their molecular structures fit into each other ● When an antibody collides (randomly) with a foreign cell that possesses non-self antigens with a complementary shape, it binds with one of the antigens When this occurs, the two molecules combine to form an antigen-antibody complex HINGE PROTEIN CONSTANT REGIONS LIGHT CHAIN DISULFIDE BRIDGE HEAVY CHAIN ANTIGEN ANTIBODY ANTIGEN-ANTIBODY COMPLEX As seen in the diagram above, antibodies have at least two antigen-binding sites This means they can bind to more than one bacterium or virus at the same time This cause groups of the same pathogens to become clumped together This process is known as agglutination The binding of antibodies to the antigens either neutralises the pathogen or acts like a marker to attract phagocytes to engulf and destroy the pathogens Due to agglutination, phagocytes can often phagocytose many pathogens at the same time, as they are all clumped together Downloaded by Ogheneyoma Adjekpovu ([email protected]) Pathogen enters body. Releases chemicals. Phagocytosis occurs. Becomes antigen presenting cell. Causes T-lymphocytes to bind to the antigen on the host cell. Stimulates the production of T-helper cells, T-memory cells and T-killer cells. T-helper cells stimulate production of B-lymphocytes. Stimulates production of plasma cells and memory cells. B lymphocytes bind to the antigen presenting cell and engulfs it by endocytosis. Produces memory and plasma cells. Plasma cells can secrete antibodies, memory cells stay in blood for faster secondary immune response. This document is available free of charge on StuDocu.com Downloaded by Ogheneyoma Adjekpovu ([email protected])