C: The Immune System

• Antigens are usually proteins
• antigens are molecules that are usually found on the cell surface membrane that can generate an immune response. they allow the immune system to recognise:

  • pathogens
  • abnormal body cells - such as tumours
  • toxins
  • cells from other organisms of the same species
• phagocytes are a type of white blood cell that is involved in the non specific immune response
• stages of phagocytosis

  1. phagocytes recognise foreign surface antigens
  2. phagocyte engulfs pathogen
  3. pathogen is now contained in a phagocytic vacuole
  4. lysosome fuses with phagocytic vacuole and releases lysozymes which break down pathogen
  5. phagocyte resents pathogen antibodies on its surface
• lymphocytes are a type of white blood cell
• T cells are produced in the bone marrow and mature in the thymus
• T cells bring about the cellular response
• stages of the cellular response:

  1. T helper cell binds to complimentary antigens on antigen presenting phagocyte
  2. this stimulates the T helper cells to divide rapidly by mitosis
  3. these cells can then release chemical signals to stimulate cytotoxic T cells, B cells and phagocytes
• 1. T cells can differentiate into T helper cells or T killer cells
• cytotoxic T cells kill abnormal and foreign cells
• B cells bring about the humoral response
• B cells are a type of white blood cell that is covered in antibodies
• Antibodies are proteins produced by the immune system in response to a foreign antigen
• Antibodies are made of two heavy chains and two light chains joined by disulphide bridges As well as two antigen binding sites which form the variable region.

  
• When the antibodies on B cells binds to a complimentary antigen it is activated - this is called colonial selection
• activated B cells divide by mitosis into plasma cells and memory cells - this is called colonial expansion
• Plasma cells are identical to the B cells and secrete lots of antibodies specific to the antigen called monoclonal antibodies
• monoclonal antibodies can bind to the complimentary antigens of the pathogens surface to form a antigen-antibody complex
• agglutination is when pathogens become clumped together because the antibodies have two binding sites so can bind to two pathogens at once. phagocytes can then engulf many pathogens at once by phagocytosis
• the primary response is slow as there aren’t enough B cells to make the antibodies needed so the infected person will show symptoms of the disease
• after being exposed to the antigen for the first time, B cells and T cells produce memory cells that will remain in the body for a long time. memory T cells will remember the specific antigen and memory B cells will remember the specific antibodies complimentary to that antigen
• The secondary response is much quicker and stronger than the primary response. Colonial selection happens faster and memory B cells are activated to divide into plasma cells that will produce complimentary antibodies to that antigen. This response kills the pathogen before the infected person starts showing symptoms.
• Active immunity is when the immune system makes its own antibodies after being stimulated by an antigen
• passive immunity is when the body is given antibodies made by another organism
• types of active immunity

  • Natural - when you become immune after being infected by a disease
  • artificial - where you become immune after being given a vaccination
• types of passive immunity

  • natural - when a baby becomes immune due to the antibodies its given by its mother
  • artificial - when you become immune after being injected with antibodies made from another organism
• active immunity takes longer to develop than passive immunity but it is much more long term as memory cells are produced whereas in passive they are not
• vaccination is when antigens are intentionally put into the body to induce artificial active immunity. live attenuated (weakened) antigens are usually longer lasting and produce a stronger response but can be unsuitable for people with weakened immune systems. inactivated antigens usually produce a shorter response and will often need boosters but are suitable for everyone
• Herd immunity is when a large proportion of a population is immune to a disease which prevents those who are not immune from catching it
• ethical issues about vaccines

  • animal testing
  • testing on humans can be dangerous
  • risk of side effects
  • deciding who is allowed vaccine first