The Human Immune System Year 11 Biology PDF

The Human Immune System Year 11 Biology Infectious Disease Infection by Pathogens Infection is the process that involves invasion of the host by a pathogen. Pathogens cause disease by damaging host cells or by producing toxins and waste products that harm the host. Pat...

The Human Immune System Year 11 Biology Infectious Disease Infection by Pathogens Infection is the process that involves invasion of the host by a pathogen. Pathogens cause disease by damaging host cells or by producing toxins and waste products that harm the host. Pathogens can gain entrance to the host through areas called “portals of entry”.  Air borne pathogens Portals of Entry may enter the mouth and nose and be absorbed across mucous membranes of the respiratory surface The skin generally provides a good protective barrier to microbes but if the skin is broken, as in a cut, then pathogens can enter. If food is contaminated then pathogens can enter via the mouth and enter the digestive Urethra in males and system females and the vagina in females can allow sexually transmitted pathogens to enter across the mucous Pathogen and host cell recognition 1. The virus binds to a specific receptor on the surface of the host cell. 2. The membrane of the virus fuses with the membrane of the host cell 3. The virus or its contents enter the host cell How a virus replicates inside a host cell 1. Virus attaches to the complementary receptor on the host cell 5 2. Virus is dismantled, releasing its DNA or RNA 3. The cell replicates the viral DNA or RNA in the nucleus 4 4. New viral components are synthesised using the host cells ribosomes 1 5. Virus is assembled and released 2 3 1 Line of Defence – st Physical Barriers The first line of defence refers to the barriers and traps to resist and prevent entry of pathogens This includes: Skin Coughing/sneezing reflex Cornea and tear ducts in the eye Earwax Mucus production Stomach acid 2nd Line of Defence – The Innate Immune System The innate immune system is the “inborn” or “natural” part of the immune system. All animals, plants and fungi have an innate immune system as it is the organisms natural defence against invading pathogens once the first line of defence has been broken. In general, the innate immune system is characterised by being: Non-specific A rapid, non-learned response A response that occurs in a very similar manner each time there is an infection The Innate Response Involves: The inflammatory response Chemicals e.g. histamine Leukocytes and phagocytosis Natural Killer Cells The Inflammatory Response When a wound occurs and pathogens enter the body, the inflammatory response occurs Chemicals are released - histamine Specialised white blood cells are released – macrophages and neutrophils Pus forms – consists of dead pathogens and dead white blood cells and other cell debris The inflammatory response is characterised by: Swelling Redness Pain Heat Histamine When a physical injury occurs, mast cells are stimulated to release a chemical called histamine. Histamine causes small blood vessels around the site of the infection to widen, allowing extra blood to flow to the site bringing with it more white blood cells to engulf the invading pathogens. Histamine also increases the permeability of the blood vessels, allowing white blood cells easier access to the wound site. White Blood Cells - Leukocytes Leukocytes are the fancy name given to white blood cells There are different types of leukocytes Neutrophils Macrophages These two types of leukocytes work together as part of the immune response, killing invading pathogens. White Blood Cells - Neutrophils Neutrophils are the first WBC to arrive at the site of infection. They engulf invading pathogens, which destroys them. Neutrophils are only able to engulf and destroy one pathogen, as they die in the process of destroying the pathogen. Dead neutrophils are one of the main components of pus. White Blood Cells - Macrophages Macrophages are larger white blood cells than neutrophils. They also engulf and destroy pathogens, however, unlike neutrophils, they can destroy pathogens over and over again. Macrophages work via a process called phagocytosis. Phagocytosis involves the cell recognising and binding to the antigens on the surface of bacteria, engulfing the bacteria and destroying it Natural Killer Cells Natural Killer Cells are not involved in the inflammatory response, however, they are another type of cell that provides protection against viruses or cancers. NK cells carry out their actions by: Recognising protein markers as “non-self” on cells that need to be destroyed Attack in numbers, punching holes through the cells they need to destroy. NK cells not only destroy pathogens, but also human cells that have become over run by pathogens or cancer. This type of cell death is Summary – Innate Immune Response If barriers and traps are considered a first line of defence to pathogens, the second line of defence includes such factors as: The inflammatory response Cells including neutrophils and macrophages (phagocytosis) and natural killer (NK) cells The innate immune system is non-specific, rapid, not learned and works in a similar way each time there is an infection The inflammatory response is a key component of the innate immune system; it is accompanied by redness and swelling. 3 rd Line of Defence – The Adaptive Immune System The adaptive (or acquired) immune response is characterised by the following: It involves specific antigens and specific pathogen recognition It is very specific, highly selective and able to detect the differences between pathogens It retains memory of the pathogen in case of subsequent infections The second exposure to the same antigen is reacted to a lot quicker by the immune system than the first exposure Adaptive Immunity The responses of the adaptive immune system are very effective, however, they are not very quick or large enough to prevent people from getting sick. When a person survives exposure to a pathogen, and they get infected with the same pathogen again, the response is much larger and more rapid and provides a greater level of protection. This means that for some one to have immunity to a specific disease, they first have to have been exposed to that specific pathogen. There are two types of adaptive immunity Passive immunity Active immunity Passive Immunity In passive immunity, antibodies provide protection This is a short lived form of immunity as antibodies have a short life span Nonetheless, passive immunity is important in several circumstances Providing protection for unborn and newly born infants who don’t yet have an active immune system Antibodies are passed from mother to child via the placenta and breast milk As an injection where immediate action is required to prevent death Protection against snakebite venom or poison Active Immunity Active immunity involves: The adaptive immune system being activated B and T cells are involved Memory cells are created and stored Active immunity comes about in two ways Exposure to the actual pathogen and hence the antigen Having a vaccination which consists of harmless antigens derived from the pathogen Adaptive Immunity - lymphocytes The most important cells involved in the adaptive immune response are two types of white blood cells called lymphocytes B lymphocytes (B cells) T lymphocytes (T cells) All blood cells come from the bone marrow B cells remain to mature in the bone marrow T cells leave to mature in the thymus (a gland in the neck) The adaptive response has close links with and works together with the innate immune response B lymphocytes At birth, humans have large numbers of different types of B cells, each with its own specific receptor molecule, which is designed to recognise one specific antigen B lymphocytes When an antigen binds to its specific receptor on the surface of a B cell, the B cell immediately divides creating many copies each containing the required receptor This is called clonal expansion These cloned B cells can do either one of two things Become plasma cells which create and secrete antibodies (protein molecules) that can bind to the specific antigen, neutralising it Become memory cells and are stored in case of a second exposure Stored B cells make the adaptive immune response much quicker the second time it is exposed to the same pathogen or antigen T lymphocytes Each person has around 10 million different T cells each specific to one antigen. T cells are part of the cell-mediated immune response unlike B cells which just release antibodies. On the surface of T cells, there are receptors that are specific to only one type of antigen Major Histo-compatability Complex The Major Histo-compatability Complex (MHC) is a set of protein markers found on the surface of cells. These proteins are coded for by genes so are unique to an individual There are two types of MHC’s “self” proteins that are unique to the individual (MHC 1) Short sections of protein antigens from pathogens “non-self” (MHC 2) It is the MHC 2 proteins that enable bonding with the T cells in the adaptive immune response MHC and T-cell Binding Once a macrophage destroys a pathogen through phagocytosis, it wears and displays the pathogens antigens on its cell membrane The antigens bind to the macrophages MHC proteins forming MHC2 markers The T cells bind to the antigen fragments on the MHC, activating the T cells. Types of T-cells There are different types of T cells all with different roles in the adaptive immune response Killer T cells – AKA Cytotoxic T cells Identify infected cells wearing MHC + antigen and secrete an enzyme to kill them Helper T cells Bind to MHC antigens and produce identical copies (clones) with the specific receptor to match the antigen. Multiple functions Activate the Killer T cells Become memory T cells for subsequent exposure to the antigen Activate specific B cells to produce the required antibody Memory T cells Stored in a memory bank to be called upon again if a subsequent exposure to the pathogen occurs Summary – Adaptive Immune Response The adaptive immune response is the third line of the body’s defence and is characterised by the following: Specificity i.e. recognition of a specific antigen/pathogen The second or subsequent exposure to the same antigen is quicker and larger due to the memory of the old antigen The Major Histo-compatability Complex (MHC) is a set of surface markers that are involved in self and non-self (antigen) recognition. Some cells of the immune system display sections on their surface attached to the MHC. B and T lymphocytes are the main cells of the adaptive immune response. They both recognise specific antigens. B cells work by releasing antibodies that bind to antigens T cells work by direct cell action releasing chemicals that destroy infected cells B and T memory cells, specific for a particular antigen, accumulate after exposure to the antigen and are responsible for the quicker and larger response after the second exposure to the antigen. Active immunity is where the adaptive immune response is activated either naturally or by vaccination. Passive immunity is brought about by the acquisition of antibodies either naturally (e.g. breast milk) or by injection (e.g.