(3) T lymphocytes and cell-mediated immunity

• lymphocytes are produced by stem cells in the bone marrow
• the two types of lymphocytes are :

  • B lymphocytes (B cells)
  • T lymphocytes (T cells)
• B lymphocytes (B cells) :

  • mature in the bone marrow
  • associated with humoral immunity - bodily fluids
• humoral immunity = immunity involving antibodies that are present in body fluids or 'humour' such as blood plasma
• T lymphocytes (T cells) :

  • mature in the thymus gland
  • associated with cell-mediated immunity
• cell-mediated immunity is immunity that involves body cells
• thymus gland is found between the lungs/ in front of the heart
• Cell-mediated immunity :

  • immature T lymphocytes leave the bone marrow to mature in the thymus
  • mature T lymphocytes have specific receptor proteins called T cell receptors
  • these receptors are found on their surface that bind to complementary antigens presented on the surface of a cell
• mature T lymphocytes have specific receptor proteins called T cell receptors
• T cell receptors bond to complementary antigen presented on the surface of a cell
• (1) in bone marrow immature T cells divide by mitosis
• (2) in the thymus, each T cell matures :

  • production of T cell receptors
  • T cell receptors in cell surface membrane
• (3) mature T cells :

  • mature T cells, each with a different T cell receptor
• T lymphocytes are activated when they encounter their specific antigen that is being presented by one of the host's cells
• anitgen presented host cell might be a macrophage or a body cell that has been invaded by a pathogen and is displaying the antigen on its cell surface membrane
• activated T lymphocytes divide by mitosis to increase in number - clonal selection
• activated T lymphocytes differentiate into two main type of T cells :

  • helper T cells
  • cytotoxic T cells
• cytotoxic T cells are also known as killer T cells
• Helper T cells :

  • initiate a cascade of immune responses
• Cytotoxic T cells :

  • kill infected cells or abnormal cells
• body can't save cells that are infected
• the receptors on each T cell respond to a single antigen and are specific to that antigen
• Helper T cells assist other white blood cells in the immune response
• T lymphocytes response :

  • pathogens invade the body cells or are taken in by phagocytes
  • the phagocyte places antigens from the pathogen on its cell surface membrane
  • receptors on a specific helper T cell fit exactly onto these antigens
  • this attachment activates the T cell to divide rapidly by mitosis and form a clone of genetically identical cells
  • clones T cells release cytotoxins
• Clones T cells :

  • develop into memory cells to enable a rapid response to future infections by the same pathogen
  • stimulate phagocytes to engulf pathogens by phagocytosis
  • stimulate B cells to divide, mature and secrete their antibody
  • activate cytotoxic T cells which destroy virus infected cells and tumour cells
• cytotoxic T cells produce a protein called perforin
• cytotoxic T cells kill abnormal cells and body cells that are infected by pathogens
• Perforin Function :

  • makes holes in the cell surface membrane
  • these holes mean the cell membrane becomes freely permeable to all substances
  • results in cell death
• cell surface membranes maintain the integrity of cells and hence their survival
• the action of T cells is most effective against viruses because viruses replicate inside cells
• as viruses use living cells in which to replicate, sacrifice of body cells prevent viruses multiplying and infecting more cells
• 1. infected body cell with antigens displayed
  2. helper T cell divides by mitosis
  3. secretes cytokines that stimulate B cells to divide and form plasma cells and memory cells
  4. vacuoles in killer T cell contain toxins
  5. killer T cells punch holes in membrane of infected body cell
  6. toxins being secreted by killer T cell into infected body cell
  7. hole in membrane of infected cell