2.5 Cell Recognition and the Immune System

• A pathogen is a microorganism that can cause disease.
• An antigen is a molecule that triggers an immune system by lymphocytes.
• An antibody is a protein produces by B-Lymphocytes in response to the appropriate antigen. They are specific to a particular antigen.
• Immunity is the means by which the body protects itself from infection.
• The two types of defence mechanisms are non-specific and specific.
• Non-specific defence mechanisms - the response is immediate and the same for all pathogens.
• Specific defence mechanisms - the response is slower and specific to each pathogen.
• The two types of non-specific defence mechanisms are physical barriers (e.g. skin) and phagocytosis.
• The two types of specific defence mechanisms are Cell-mediated response (T-Lymphocytes) and Humoral response (B-Lymphocytes).
• The immune system recognises transplant cells as non-self so it attempts to destroy the transplant. Immunosuppressant drugs reduce the level of immune response that occurs.
• 1. In foetus, lymphocytes collide with other cells.
  2. Infection is rare in the womb, so lymphocytes collide with self-cells.
  3. Some lymphocytes fit the body cell receptors exactly - these die/are suppressed.
  4. Only remaining lymphocytes are those that fit with foreign material.
  5. In adults, lymphocytes in the bone marrow only encounter self antigens. Any that bind to self-antigens undergo apoptosis (programmed cell death).
  6. No clones of the anti-self cells will appear in the blood.
  7. This is how the immune system knows not to attack body cells.
• Give two ways in which pathogens can cause disease.


  -They can release toxins.

  -They can kill cells.
• Putting bee honey on a cut kills bacteria. Honey contains a high concentration of sugar.

  Use your knowledge of water potential to suggest how putting honey on a cut kills bacteria.

  The water potential of honey is lower than the water potential of a bacterium, so the water would travel into the honey by osmosis and leave the bacterium, causing it to shrink and die.
• 1. Phagocytes are attracted by chemoattractants (chemical product of pathogens).
  2. The phagocyte chases then engulfs the pathogen.
  3. This forms a phagosome (vesicle).
  4. Lysosomes then migrate towards the phagosome.
  5. These lysosomes then release hydrolytic enzymes into the phagosome to digest the pathogen.
  6. Useful products produced are absorbed into the lysosome.
  7. The phagocyte then become an antigen presenting cell to trigger an immune response.
  8. This is phagocytosis.
• B-Cells mature in the bone marrow.
• T-Cells mature in the thymus gland.
• Both B-Cells and T-Cells are produced in the bone marrow.
• B-Cells are associated with humoral immunity involving antibodies that are present in 'humours' e.g. blood plasma.
• T-Cells are associated with cell mediated immunity, that is immunity involving body cells.
• Cytotoxic T-Cells - destroy the antigen carrying cells by attaching to them and releasing the chemical perforin to kill them.
• How does perforin kill antigen carrying cells?

  Perforin (a glycoprotein) makes holes in the cell-surface membrane so it becomes freely permeable and dies as a result.
• Helper T-Cells - attract and stimulate macrophages and promote the activity of other T and B cells.
• Cytotoxic T-Cells (Tc) kill abnormal cells and cells infected by pathogens.
• Perforin is a glycoprotein.
• The different types of T-Cell are Cytotoxic T-Cells (Tc) and T-Helper Cells (Th).
• Antibiotics work by stopping bacteria reproducing by binary fission/stop cell from forming which kills them.
• Compare the action of perforin to the actions of antibiotics.


  Perforin creates holes in the cell-surface membrane so it becomes freely permeable and dies as a result. However antibiotics stop bacteria reproducing. Perforin works on infected body cells, antibiotics only work on bacteria.
• Examples of antigen presenting cells:

  • Phagocytes which have engulfed a pathogens and has presented the pathogens antigens on its surface.
  • Body cells invaded by a virus.
  • Transplanted cells.
  • Cancer cells.
• Cell-Mediated Immunity:
• Cell-Mediated Immunity:

  1. Pathogens invade body cells or are taken in by phagocytes.
  2. The phagocyte places antigens from the pathogen on its cell-surface membrane.
  3. Receptors on a specific helper T-Cell fit exactly onto these antigens.
  4. This attachment activates the T-Cell to divide rapidly by mitosis to form a clone of genetically identical cells.
  5. The cloned T-Cells:

  -develop into memory cells

  -stimulate phagocytes

  -stimulate specific B-Cells

  -activate cytotoxic T-Cells
• In Cell-Mediated Immunity, how are pathogens identified?


  Need antigen presenting cells to produce a response to pathogens.
• In Humoral Immunity, how are pathogens identified?

  Bind specifically to a pathogen.
• In Cell-Mediated Immunity, how are pathogens killed?


  Tc cells release perforin which creates holes in the membrane, making them freely permeable and kills the pathogens.
• In Humoral Immunity, how are pathogens killed?


  Antibodies produces that attach to antigens on the pathogen and destroys them.