2C - the immune system

• describe the steps of the first stage of the immune response?

  1. a phagocyte recognises the foreign antigens on a pathogen.
  2. the phagocyte's cytoplasm moves around the pathogen, engulfing it.
  3. the pathogen is now contained in a phagocytic vacuole in the phagocyte's cytoplasm.
  4. a lysosome fuses with the phagocytic vacuole. The lysozymes break down the pathogen.
  5. the phagocyte presents the pathogens antigens by sticking the antigens on its reface to activate other immune system cells.
• describe the 2nd stage of the immune response?


  • A T-cell has receptor proteins on its surface that bind to complimentary antigens presented to it by phagocytes.
  • This activates the T-cell.
• what do helper T-cells do?


  helper T-cells release chemical signals that activate and stimulate phagocytes and cytotoxic T-cells, they also secrete antibodies.
• describe the 3rd stage of the immune response?


  1. B-cells are covered with antibodies, each B-cell has a different shaped antibody on its membrane so different ones bind to different shaped antigens.
  2. when the antibody on the B-cells's surface meets a complimentary shaped antigen, it binds to it.
  3. this, and the substances releases from helper T-cells, activates the B-cell. This is clonal selection.
  4. the activated B-cell divided into plasma cells.
• describe the 4th stage of the immune response


  • plasma cells are identical to B-cell.
  • They secrete lots of antibodies specific to the antigen. Monoclonal antibodies.
  • they bind to the antigens on pathogen's surface form antigen-antibody complexes.
  • antibody has 2 binding sites so binds to 2 pathogens at the same time.
  • this causes pathogens to clump together (agglutination).
  • phagocytes bind to antibodies and phagocytise many pathogens at once.
  • this leads to destruction of pathogens carrying this antigen.