Immune system

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Antigens

  • Antigens are molecules that generate an immune response when detected by the body, they’re usually found on the surface of all cells.
  • Antigens not usually found in the body are known as foreign antigens, they are the antigens the immune system responds to.
  • Antigens also allow the immune system to identify pathogens- organisms that cause disease, abnormal body cells- cancerous and pathogen infected toxins- poisonous molecules that are not cells some are produced by bacteria, and cells from other individuals from the same species which can lead to the rejection of transplanted organs if drugs aren’t taken to suppress the recipient’s immune system.
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Antibodies

  • Antibodies are proteins. Its specificity depends on its variable regions which form the antigen binding sites. Each antibody has a variable region with a unique tertiary structure that is complementary to one specific antigen. All antibodies have the same constant region. Image result for antibodies
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Phagocytosis

  • A phagocyte is a type of white blood cell that carries out phagocytosis, eg a macrophage. They’re found in white blood cells and in tissues and are the first cells to respond to an immune system trigger inside the body.
  • Phagocyte recognises the foreign antigens on a pathogen
  • Cytoplasm of phagocyte moves around the pathogen and engulfs it
  • The pathogen is contained in the phagocytic vacuole which is in the cytoplasm of the phagocyte.
  • A lysosome fuses with the phagocytic vacuole and breaks the pathogen down
  • The phagocyte then presents the pathogens antigens by sticking the antigens on its surface to activate other immune system cells.
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T-Cells

  • T-cells is a type of white blood cell, that has receptor proteins on its surface which bind to complementary antigens presented to it by phagocytes which activates the T-Cells.
  • There are different types of T-Cells: helper T-cells release chemical signals that activate and stimulate phagocytes, cytotoxic T-cellskill abnormal and foreign cells. Helper T-cells activate B-cells which secrete antibodies.
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B-Cells

  • Antibodies are proteins that bind to antigens to form an antigen-antibody complex.
  • B-cells are a type of white blood cell covered with antibodies.
  • Each B-cell has a different shaped antibody on its membrane, so different ones bind to different shaped antigens.
  • When the antibody on the B-cell meets a complementary shaped antigen it binds to it. This and substances released from helper T-cells activates the B-cells in a process called clonal selection where activated B-cells divide into plasma cells.
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Antibody Production

  • Plasma cells are identical to the B-cells and secrete loads of antibodies specific to the antigen. These are monoclonal antibodies.
  • They then bind to the antigens on the surface of the pathogens and form lots of antigen-antibody complexes.
  • Antibodies have two binding sites so two pathogens can be binded at once, and causes agglutination- when pathogens become clumped together.
  • Phagocytes then bind to the antibodies and phagocytose many pathogens at once which leads to the destruction of pathogens carrying the antigen around the body.
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Cellular and Humoral response

  • Cellular- T-cells and other immune system cells that they interact with eg phagocytes, form the cellular response
  • Humoral- B-cells, clonal selection and production of monoclonal antibodies form the humoral response.
  • Both are needed to remove a pathogen from the body and the responses interact with each other, eg T-cells help to activate B-cells and antibodies coat pathogens making it easier for phagocytes to engulf them.
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Primary and secondary immune response

  • Primary response is when an antigen enters the body for the first time. It is slow because there aren’t many B-cells to make the antibody needed to bind to it. Eventually enough of the right antibody will be made but the body will show symptoms.
  • After being exposed T and B cells produce memory cells which stay in the body for a long time.
  • Memory B-cells record the specific antibodies needed to bind the antigen.
  • Memory T-cells remember the specific antigen and recognise it the second time round
  • Secondary response the same pathogen enters the body. The response is quicker and stronger. Clonal selection happens faster. Memory B-cells are activated and divide into plasma cells that produce the correct antibody to the antigen.
  • Memory T-cells are activated and divide into the correct type of T-cells to kill the cell carrying the antigen. The pathogen is usually removed before symptoms show.
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Active and Passive immunity

  • Active immunity is what you get when your immune system makes its own antibodies after being simulated by an antigen.
  • Natural: when you are immune after catching a disease
  • Artificial: when you are immune after having a vaccine containing a harmless dose of the antigen
  • Passive immunity is what you get after given antibodies from another organism
  • Natural: when a baby becomes immune after receiving antibodies from its mother
  • Artificial: when you become immune after being injected with antibodies from someone else.
  • Active immunity requires exposure to antigen, passive doesn’t, active immunity takes a while for protection to happen, passive is immediate, memory cells produced in active, not in passive.
  • Protection is long term in active immunity because antibodies are produced in response to complementary antigen being present in the body, whereas protection is short term in passive because antibodies break down.
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Vaccination and Ethics

  • Always contain antigens that cause the body to produce memory cells against a particular pathogen without the pathogen causing disease. They may be injected or taken orally- disadvantage of orally is that could be broken down by enzymes in the gut or molecules are too large to be absorbed into the blood.
  • Booster vaccines can be given to make sure memory cells are produced.
  • They allow you to become immune without having symptoms.
  • They protect individuals and those around due to herd immunity as they reduce the occurrence of the disease.
  • Ethical issues: Tested on animals before humans- people disagree with animal testing, animal based substances may be used to produce a vaccine which people may disagree with. Testing on humans is risky as volunteers may put themself at risk of catching a disease because they think they are fully protected. People may not take the vaccine due to risk of side effects but are still protected because of herd immunity, which people see as unfair.
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Antigenic variation

  • Antigenic variation is when pathogens change their surface antigens.
  • When you are infected a second time the memory cells produced will not remember the different antigens so the immune system has to carry out the primary response again with new antigens
  • Antigenic variation makes it hard to form vaccines against some pathogens for the same reason
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Monoclonal antibodies

  • Antibodies produced from a single group of genetically identical B-cells.
  • Monoclonal antibodies can be made that bind to anything that you want and will only bind to this molecule. This can be used to treat illnesses and medical diagnosis.
  • E.G: used to target cancer cells by making monoclonal antibodies that bind to tumour markers on cancer cells. Anti-cancer drugs can also be attached to the antibodies so when the antibodies come in contact with the cancer cells they will bind to the tumour markers. This reduces the side effects as the drug only accumulates near specific cells as the drug only accumulates in the body where there are cancer cells.
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Ethics of monoclonal antibodies

  • Ethical issues are that they often involve animal right issues as animals are used to produce the monoclonal antibodies and some people disagree with the use of animals in this way.
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ELISA test

  • Allows you to see if a patient has antibodies to a certain antigen and antigen to an antibody.
  • It can be used for medical diagnosis to test for infections or allergies.
  • In the test an antibody is used which has an enzyme attached. The enzyme reacts with the substrate and produces a coloured product.
  • If there’s a colour change the antigen or antibody is present in the sample tested. In some ELISA test the quantity can be worked out by the intensity of the colour change.
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Direct ELISA

  • Direct ELISA uses a single antibody that is complementary to the antigen.
  • The antigens from a patient sample are bound to a well in a well plate. A detection antibody that is complementary to the antigen of interest is added. If the antigen is present it will be immobilised on the inside of the surface of the well and a detection antibody will bind to it
  • The well is then washed out to remove any unbound antibody and the substrate solution is added.
  • If the detection antibody is present the enzyme reacts with the substrate and produces a colour change which is a positive result for the presence of the antigen.
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Indirect ELISA

  • Indirect ELISA uses two antibodies and can be used to see if a patient has antibodies to a certain disease eg HIV.
  • HIV antigen is bound to the bottom of a well in a well plate
  • A sample of patient’s blood plasma is added to the well. The HIV-specific antibodies in the plasma will bind to the HIV antigen at the bottom of the well. The well is then washed out to remove unbound antibodies
  • A secondary antibody is then added that has an enzyme attached which binds to the HIV-specific antibody. The plate is then washed out again to remove unbound secondary antibody
  • A solution is then added to the well which contains a substrate and reacts with the enzyme attached to the antibody and produces a coloured product, if a colour change occurs the patient has HIV-specific antibodies and their blood is infected with HIV.
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HIV and AIDS

  • HIV is a virus that affects the immune system and leads to AIDS which is a condition where the immune system deteriorates and eventually fails.
  • HIV infects and kills helper T-cells which act as host cells for the virus. Without enough helper T-cells the immune system is unable to have an effective response because other immune system cells don’t behave as they should. AIDS is developed when the amount of helper T-cells drops to a low level.
  • During the infection period, HIV replicates rapidly and the infected person experiences many flu like symptoms. After this the replication drops to a lower level called the latency period which can last for years and no symptoms will be experienced.
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Symptoms of AIDS

  • Minor infections of mucous membranes and recurring respiratory infections
  • As AIDS progresses the number of immune system cells decrease and patients become more susceptible to more serious infections such as diarrhoea and bacterial infections.
  • During the late stages of AIDS patients can develop a range of serious infections including toxoplasmosis of the brain and candidiasis of the respiratory system.
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HIV structure

  • Spherical structure.
  • Made up of a core contraining genetic material, and proteins eg reverse transcriptase.
  • Has a capsid which is an outer coating protein, and an envelope which is an extra outer layer.
  • Sticking out of the envelope are lots of copies of an attachment protein which helps the HIV attach to the host helper T-cell.
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HIV Replication

  • Attachment protein attaches to a receptor molecule on the host helper t-cell.
  • Capsid is released into the cell and uncoats and releases the RNA into the cells cytoplasm.
  • reverse transcriptase is used to make a complementary strand of DNA from the viral RNA template which is then used to make double stranded DNA which is inserted into the human DNA.
  • Host cell enzymes are used to make viral proteins from the viral DNA that was inserted into the human DNA.
  • These viral proteins are then assembled into new viruses and bud from the cell and go on to affect other cells.
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Controlling HIV infection

  • Antiviral drugs used to slow down the progression of HIV infection and AIDS in the infected person.
  • Best method of controling is reducing the spread of HIV
  • Its spread through unprotected sex, bodily fluids, and from a HIV positive mother to her fetus.
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Antibiotics and viruses

  • Antibiotics kill bacteria by interfering with their metabolic reactions.
  • They target bacterial enzymes and ribosomes used in the reactions as bacterial enzymes and ribsomes are different from human ones.
  • Antibiotics only kill bacterial enzymes and ribosomes so they dont damage human cells.
  • Antibiotics cant inhibit human viruses because they use human enzymes and ribosomes to replicate so antibiotics cant inhibit them as they dont target human processes.
  • Most antiviral drugs are used to target viral enzymes eg HIVs reverse transcriptase.
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