3.2.4 Immune response

• Pathogen 


  The cause of an infectious disease
• Antigen


  A foreign protein/glycoprotein that triggers an immune response leading to the production of specific antibodies
• Barriers from pathogens

  Skin

  Stomach acid

  Muscle lining
• Phagocytosis

  Immediate response

  1. Phagocyte recognises foreign antigens on pathogen
  2. C.S.M and cytoplasm engulf the pathogen in a phagocytic vesicle
  3. Lysosomes move into vesicle and membranes fuse together
  4. Lysosomes digest the pathogen in hydrolysis reactions
  5. Waste products are expelled and cell becomes antigen presenting
• Cellular response

  After phagocytosis

  1. Specific Th cells are activated by the antigen presenting cell
  2. Th cells divide by mitosis to form clones of more specific Th cells, the Th cells release chemicals called cytokines
  3. This activates: cytotoxic t cells which kill infected body cells through perforin and cell lysis; B cells; attracts phagocytes
  4. Specific T memory cells are produced
• Humoral response

  At the same time as the cellular response

  1. Specific B cells are activated by cytokines
  2. B cells divide by mitosis to form clones of plasma cells and memory B cells
  3. Plasma cells make and secrete monoclonal antibodies
  4. These are secreted into the blood, where they encounter a pathogen.
  5. Antibodies bind to the antigens and cause agglutination
  6. Phagocytosis then occurs to remove the pathogen
• Primary immune response

  The first time a pathogen with a specific antigen enters your body, the system will respond and develop memory cells
• Secondary immune response

  The same pathogen enters. As you have memory cells for that specific antigen you are immune.

  1. Pathogen enters, phagocytosis occurs
  2. Antigen presenting cell present antigen to specific B memory cells
  3. B memory cells divide rapidly to form more plasma cells
  4. Antibodies are made faster and in larger quantities - faster than the pathogen can multiply in the body
  5. You are immune and will not get the disease
• Antigenic Variation

  Pathogens can change the shape of their antigens regularly
• Vaccines can be made of

  A dead pathogen

  A weak/attenuated pathogen (not capable of reproducing)

  The antigen alone
• How vaccines give immunity to disease

  Vaccine given containing the antigen, this causes specific B cells to be activated for the first time. The first clone of B memory cells are produced.

  A booster is given later with more of the same antigen. The specific B cells are activated again, dividing to form more memory cells.

  Immunity is acquired
• Herd immunity

  A high percentage of the population is vaccinated, reducing the chances of an infected person coming into contact with an unvaccinated person.
• Active immunity

  Long term immunity developed after being exposed to the antigen. Specific T and B memory cells produced

  Natural - after having the disease

  Artificial - Vaccination
• Passive immunity


  Short term after being given the antibodies from another person. No T or B memory cells produced.

  Natural - Mother to baby

  Artificial - Injected with antibodies
• Uses of monoclonal antibodies

  Produced by clones of plasma cells

  Used in -

  • Attaching drugs - binds to a specific antigen, drug is activated to kill the cell (often used in treating cancer)
  • Medical diagnosis - ELISA testing, detects the presence of a specific antigen in a sample
• Direct ELISA

  1. Antigen from the sample added - binds to plate
  2. mAb with enzyme attached is added, binds to antigen
  3. Substrate of enzyme added
  4. If the antigen was present a colour change will be seen

  
• Indirect ELISA

  1. Antigen bound to plate
  2. Antibody from sample added - binds to antigen
  3. mAb with enzyme attached added - binds to first antibody
  4. Substrate of enzyme added, if positive a colour change can be seen

  
• Sandwich ELISA

  1. Antibody bound to plate
  2. Antigen from sample added - binds to antigen
  3. Specific antibody added
  4. Antibody with enzyme binds - if positive a colour change can be seen

  
• Important note for ELISA testing

  Wash wells between steps.

  This will -

  • Remove unbound antibodies
  • Remove unbound antigens
  • Remove unbound antibodies with enzymes attached which could give a false positive results