Immunity

• The immune system, antibodies and vaccination are covered in Topic 11
• Phagocytosis


  1. Phagocytes (macrophages and neutrophils) travel in blood, squeeze out of capillaries, engulf and digest pathogens

  2. Damaged cells and pathogens release cell signalling chemicals (cytokines) which attract phagocytes

  3. Opsonin proteins attach to pathogens to mark them for easier phagocytosis

  4. Phagocytes have receptors that attach to chemicals on pathogen surface

  5. Pathogen surrounded and engulfed into phagosome

  6. Phagosome fuses with lysosome containing hydrolytic enzymes to digest pathogen

  7. Soluble useful molecules absorbed, indigestible parts expelled
• Antigen presenting cells


  Phagocytes that present antigens of digested pathogens on their surface
• Cell-mediated immune response


  1. T helper cells bind to antigen presenting cells

  2. T helper cells divide rapidly by mitosis (clonal expansion)

  3. T helper cells differentiate into more T helper cells, T memory cells, T regulatory cells, cytotoxic T cells

  4. Cytotoxic T cells release perforin to create pores in infected/abnormal cells, causing cell death
• Humoral immune response


  1. T helper cells bind to and activate B cells

  2. Activated B cells divide by mitosis (clonal expansion)

  3. B cells differentiate into memory B cells and plasma cells

  4. Plasma cells produce antibodies that bind to antigens
• Primary immune response


  First exposure to a pathogen, takes time to produce enough antibodies
• Secondary immune response


  Faster antibody production due to memory cells from previous exposure
• Antibodies


  Proteins produced by plasma cells that bind to antigens
• Antibodies


  • Globular proteins with 2 light and 2 heavy polypeptide chains
  • Binding sites with unique shape complementary to antigen
  • Can agglutinate, opsonise or act as antitoxins
• Monoclonal antibody production


  Immunise animal, collect B cells, fuse with tumour cells, culture hybridomas, screen and clone to produce identical antibodies
• Uses of monoclonal antibodies


  • Medical treatment, medical diagnosis, pregnancy tests, drug tests
• Passive immunity


  Receiving antibodies directly, provides temporary immunity
• Active immunity


  Producing own antibodies after exposure to pathogen or vaccine
• Vaccines can induce active artificial immunity by exposing to weakened/attenuated pathogens
• Pathogens can mutate, leading to antigen variability that reduces vaccine effectiveness
• Epidemic


  Disease spreading rapidly within a country
• Pandemic


  Disease spreading rapidly globally
• Mass vaccination programs and herd immunity can prevent disease spread