5.1 - Defence Mechanisms

• immunity


  body's defence mechanisms overwhelm the pathogen & individual recovers from disease

  so the body's defences are better prepared for a second infection by the same pathogen so it can kill it before causing harm
• non-specific defence mechanisms


  immediate response

  the same for all pathogens

  physical barriers (skin, cilia)& phagocytosis
• specific defence mechanisms


  slower response

  specific to each pathogen

  cell mediated responses (t-lymphocytes) & humoral response (b-lymphocytes)
• resistance


  ability to ward off disease through defence mechanisms
• non-specific resistance


  protects against a broad range of pathogens

  provided by the 1st and 2nd lines of defence
• specific resistance


  specific to a particular pathogen

  3rd line of defence

  immune response which involves antibodies
• antibodies


  proteins that identify and neutralise antigens
• antigens


  foreign or harmful substances
• what's the first line of defence


  an external barrier to stop pathogens entering the body
• what makes up the first line of defence


  skin

  coughing & vomiting

  tears

  mucous membranes and their secretions
• how the skin prevents pathogens from entering the body


  have a low pH which is unfavourable to the growth of bacteria

  have populations of harmless microbes on it that prevents growth of most pathogenic microbes
• coughing and vomiting as a line of defence


  expulsione reflexes help remove pathogens from the body
• tears as a line of defence


  contain antimicrobial substances & lysozymes

  washes contaminants from the eyes
• mucous membranes & their secretions as a first line of defence


  found in the lining of the respiratory, urinary, reproductive and gastrointestinal tracts

  Microorganisms are trapped in sticky mucus and expelled by cilia
• what's the second line of defence

  defence mechanisms that operate inside the body to inhibit or destroy any pathogen
• complement system


  plasma proteins work together to bind pathogens and induce an inflammatory response to help fight infection
• second lines of defence


  complement system

  antimicrobial substances

  inflammation + fever

  phagocytic white blood cells
• antimicrobial substances


  produce toxic proteins against certain parasites and phagocytosis
• inflammation and fever


  releases an anticoagulant and histamine which promotes inflammation
• phagocytic white blood cells


  engulf and destroy foreign material
• third line of defence


  lymphocytes launch Specific responses to the pathogen, including the production of antibodies

  b-cells

  t- cells
• B-cells


  recognise specific antigens and divide to form antibody producing clones
• T- cells


  recognise antigens and activate defensive cells
• why it's important for the body to recognise its own cells

  it enables the body to distinguish its aun tissues from foreign material so lymphocytes don't destroy the organism's own cells
• how self-recognition is achieved


  through the major histocompatibility complex (MHC)
• major histocompatibility complex


  tightly linked genes on chromosome 6 that code for MHC antigens attached to the surface of body cells
• what feature distinguishes one cell from the other

  proteins on the surface

  they have a highly specific tertiary structure so each 3D structure will be different
• non-self cells the immune system identifies


  pathogens

  cells from other organisms from the same species

  toxins

  abnormal body cells
• what self recognition failure can lead to


  autoimmune disorders where the immune system mistakenly destroys its own tissues

  e.g: type 1 diabetes
• drawback of the immune system


  it's responsible for the rejection of tissue and organ transplants as it recognises the calls as non-self so tries to destroy them
• what's done to minimise rejection in tissue/organ transplants

  donor tissue is matched to the patient's as closely as possible (usually from a relative)

  immunosuppressant drugs are administered
• why are some cancer cells detected & destroyed by the immune system


  they have surface protein the immune system recognises as foreign
• why some cancer cells go undetected


  the immune system doesn't recognise them as foreign so they continue to divide and form tumours
• how pathogens avoid detection


  by altering their surface proteins

  hiding within a host cell
• why is there a lag time between pathogen exposure and the body's defence


  one of the millions of lymphocytes will have a protein on its surface complementary to one of the proteins on the pathogen so it will be stimulated to divide to build up its numbers to destroy pathogen
• how self recognition by lymphocytes works


  in fetus millions of lymphocytes collide with other body ceIls only

  some will have receptors that fit body's own cells so they die or are suppressed

  only remaining lymphocytes are those that fit non-self so only respond to foreign material
• why infections in the fetus are rare & lymphocytes in fetus only collide with body cells


  fetus is protected by mother and placenta
• where lymphocytes are produced in adults


  in the bone marrow

  only encounter self antigens at first
• what happens to lymphocytes in the bone marrow that show immune responses to self antigens


  they undergo programmed eall death (apoptosis) before they differentiate into mature lymphocytes so no clones of anti-self lymphocytes appear in the blood - there will only be lymphocytes that respond to non-self antigens