3.5-3.8

• Pathogens


  Disease causing biological agents such as bacterium, virus, protozoan or fungi
• Non-specific defences


  • The body's first line of defence
  • Protect the body against ANY pathogen
• Epithelial cells


  • Form a physical barrier against invading pathogens
  • Closely packed in skin and inner linings of digestive and respiratory systems
  • Produce chemical secretions against invading pathogens
• Chemical secretions


  Tears, saliva, mucus and stomach acid
• Mucus produced by epithelial cells in the respiratory system traps foreign particles which may cause harm to the lungs
• Stomach acid secreted by epithelium cells lining the stomach destroys many of the microbes that have been swallowed
• Tears and saliva


  Contain an enzyme called lysozyme that destroys the cell walls of some bacteria
• Skin cells produce chemical secretions such as sweat against invading pathogens
• Inflammatory response


  • Happens when the body suffers an injury such as a cut or the invasion by microorganisms
  • Localised defence mechanism
• Mast cells


  • Present in tissues
  • Release a chemical called histamine
• Histamine


  Causes increased capillary permeability and vasodilation
• Increased blood flow


  Leads to an accumulation of phagocytes and clotting elements at the site of infection
• Phagocytes


  • White blood cells involved in the non-specific immune response
  • Recognise pathogens and destroy them by phagocytosis
• Phagocytosis


  Engulfing of pathogens and their destruction by digestive enzymes contained in lysosomes
• Cytokines


  Protein molecules that act as a signal to specific white blood cells causing them to accumulate at the site of infection
• Phagocytes release cytokines which attract more phagocytes to the area of infection
• Examples of pathogens


  • Bacterium
  • Virus
  • Protozoan
  • Fungi
• Non-specific defence


  Protects the body against any pathogen
• Epithelial cells form a physical barrier and produce chemical secretions against invading pathogens
• Inflammatory response


  Localised defence mechanism when the body suffers an injury or invasion by microorganisms
• Mast cells


  • Release histamine
• Cytokines


  Protein molecules that signal specific white blood cells to accumulate at the site of infection
• Lymphocytes


  White blood cells involved in the specific immune response
• Antigens


  Molecules, often proteins, located on the surface of cells that trigger a specific immune response
• Lymphocytes


  • Have a single type of membrane receptor specific for one antigen
  • Different from any other type of lymphocyte
• Antigen binding


  1. Leads to repeated lymphocyte division

  2. Resulting in a clonal population of identical lymphocytes
• B lymphocytes


  • Produce antibodies against antigens
  • Leads to destruction of the pathogen
• Antibodies


  1. shaped proteins with receptor binding sites specific to a particular antigen on the pathogen
• Antibody binding


  1. Inactivates the pathogen

  2. Antigen-antibody complex can be destroyed by phagocytosis
• B lymphocytes can respond to normally harmless antigens, leading to allergic reactions
• T lymphocytes


  Destroy infected body cells by recognising antigens of the pathogen on the cell membrane and inducing apoptosis
• Apoptosis


  Programmed cell death
• T lymphocytes attach to infected cells and release proteins that diffuse into the cells, causing them to produce self-destructing enzymes which cause cell death
• T lymphocytes can normally distinguish between self-antigens and non-self-antigens, but failure of immune system regulation leads to autoimmune diseases where T lymphocytes attack the body's own cells
• Autoimmune diseases


  • Rheumatoid arthritis (T lymphocytes attack cells in the joints)
  • Type 1 diabetes (T lymphocytes attack insulin producing cells in pancreas)
• Memory cells


  Long-lived lymphocytes capable of rapidly responding to a particular antigen on its reintroduction
• Secondary exposure to the same antigen


  Antibody production is greater and more rapid, so the individual does not develop the same infection again
• The human immunodeficiency virus (HIV) attacks and destroys T lymphocytes, leading to the development of AIDS and a weakened immune system vulnerable to opportunistic infections
• Opportunistic infections in AIDS

  • Pneumonia
  • Salmonella infection
• Specific immune response


  • Action of lymphocytes
  • B lymphocytes
  • T lymphocytes