Immune system

• An infection is the interaction between a pathogen and the body’s defences.
• To recover from an infection, the body’s immune system must take over the pathogen.
• Immunity is built up when the body’s defences remember a pathogen from a previous infection so they can remember how to combat it and prevent it from harming the body.
• An antigen is part of a pathogen which alerts the body to an infection and stimulates an immune response.
• Antigens are often proteins that are part of the cell-surface membrane. The presence of antigens triggers the production of an antibody as part of the body’s defence system.
• Antibodies are proteins which has a specific binding site, made by B lymphocytes. Each antibody has two identical binding sites and can therefore bind to two antigens to form an antigen-antibody complex.
• Antibodies destroy bacterial cells in the following way:

  1. They cause bacterial cells to clump together.
  2. They then stimulate phagocytes to engulf the bacterial cells to which they are attached.
• Antibodies are made up by four polypeptide chains:

  Heavy chains – Long and make up one pair.

  Light chains – Short and make up the other pair.
• Lymph Nodes – Contain lymphocyte cells of the immune system which work to recognise and destroy invading microorganisms.
• Respiratory System – Cells with cilia on their surface line the airways and clear away mucus that traps inhaled germs.
• Skin – Provides a barrier against germs.
• Stomach + Intestines – Stomach acid kills harmful bacteria, and antibodies secreted by intestinal cells attack viruses and other microorganisms travelling through the gastrointestinal tracts.
• White Blood Cells – Circulating white blood cells attack germs in the blood and other body tissues.
• Spleen – The spleen helps to protect the body against bacterial infections.
• T Lymphocytes

  Cell mediated response, cells attack other cells.

  Mature in the thymus gland.
• The stages of a T-lymphocyte response are:

  1. Body cells are invaded, or phagocytes engulf a pathogen.
  2. The body cell or phagocyte places antigens from the pathogen on its cell-surface membrane
  3. Receptors on a specific helper T lymphocyte fit exactly onto the antigens.
  4. This attachment activates the T cells to divide rapidly by mitosis and form clones.
  5. The cloned T cells stimulate phagocytes to begin phagocytosis, stimulate B lymphocytes to secrete antibodies, and activate cytotoxic T cells.
• Cytotoxic T cells kill abnormal cells and body cells that are infected by pathogens, by producing a protein called perforin that makes holes in the cell-surface membrane. The sacrifice of body cells prevents viruses multiplying and infecting more cells.
• B Lymphocytes:

  Humoral response, a result of antibodies being released into the body’s fluids.

  Mature in bone marrow.
• Each B cell produces an antibody that responds and attaches to one specific antigen. The antigen enters the B cell by endocytosis and gets presented on its surface. T helper cells bind to the processed antigens and stimulate the B cell to divide by mitosis to form clones which secrete antibodies.
• Clones of B cells can develop into:

  Plasma cells – which secretes antibodies into blood plasma to destroy an antigen.

  Memory cells – which are responsible for a secondary immune response so if the pathogen appears again, they can divide into more memory cells or plasma cells at a higher scale and faster rate than the first infection.
• Each clone of a B cell secretes identical antibodies for a specific antigen which are referred to as monoclonal antibodies.
• Monoclonal antibodies can be used to treat cancers in 2 ways:

  1. They attach to the surface of cancer cells and block chemical signals causing uncontrolled growth
  2. They attach to a radioactive drug and then attach to a cancer cell to kill it.
• Monoclonal antibodies can be used for medical diagnosis by interacting with particular antigens, making it possible to measure the level of specific substances in the blood.
• Monoclonal antibodies can also be used for pregnancy testing by having the ability to attach to human chorionic gonadotrophin (hCG) that is found in pregnant urine. The antibody-colour complex can move across the pregnancy test until it is trapped by another antibody in the test causing a coloured line.
• Lymphocytes are preexisting in the body, they are not produced specifically in response to an infection. There are very few of each type of lymphocyte in the body so when an infection occurs, the specific type of lymphocyte already present is stimulated to build up its numbers to a level where it can be effective in destroying the infection.
• Lymphocytes need to be able to make a distinction between the body’s own cells and those that are foreign because if they couldn’t do this the body would destroy its own tissues.
• Lymphocytes can recognise the body’s own cells because they are present in foetuses, where they are constantly colliding with other cells and collide with almost the entire of the body’s tissue. Some lymphocytes will have receptors that exactly fit the body’s cells and these lymphocytes will either die or be suppressed so the only remaining lymphocytes are those that fit foreign material. Infection is rare in foetuses because they are protected from the outside world.
• In adults, lymphocytes that are produced in the bone marrow initially only encounter self-antigens.  Any lymphocytes that show an immune response to these self-antigens undergo programmed cell death.
• Phagocytes are a type of white blood cell which ingest and destroy pathogens using a process called phagocytosis.
• Phagocytosis

  STEP 1:

  Chemical products of pathogens or dead, damaged or abnormal cells act as attractants. These attractants cause the phagocyte to move towards the pathogen.
• Phagocytosis

  STEP 2:

  Phagocytes have several receptors on their cell-surface membrane that attach to chemicals on the surface of the pathogen.
• Phagocytosis

  STEP 3:

  The phagocytes surround the pathogen to form a vesicle called the phagosome.
• Phagocytosis

  STEP 4:

  Lysosomes move towards the phagosome and fuse with it.
• Phagocytosis

  STEP 5:

  Enzymes called lysozymes are released by the lysosome. The lysozymes destroy ingested bacteria by breaking down their cell walls.
• Phagocytosis

  STEP 6:

  The soluble products from the breakdown of the pathogen are absorbed into the cytoplasm of the phagocyte and then some of them are released out of the cell by exocytosis.