T- lymphocytes

• 1. Lymphocyte Response


  Specific Immune Response: T Lymphocytes
• Lymphocytes and antibodies


  • Provide the third line of defence against pathogens
  • Specific immune responses are slower but more effective than non-specific immune responses
• Lymphocytes


  • A type of white blood cell
  • Smaller than phagocytes
  • Have a large nucleus that fills most of the cell
  • Produced in the bone marrow before birth
  • Travel around the body in the blood
• Types of lymphocytes


  • T-lymphocytes (T cells)
  • B-lymphocytes (B cells)
• Maturation of T-lymphocytes


  1. Immature T-lymphocytes originate in the bone marrow

  2. They move to the thymus gland in the chest, which is where they mature

  3. During the process of maturation T lymphocytes (T cells) gain specific cell surface receptors called T cell receptors (TCRs)

  4. A small number of T cells have the same TCRs, these genetically identical cells are called clones

  5. T cells within each clone differentiate into different types of T cell: T helper cells, T killer cells and T regulator cells
• T cells


  • There is a very large number of different T cells with different TCRs
  • This variation allows the T cells to recognise a wide range of foreign antigens
  • Foreign antigens can be found on the surface of microorganisms, their cell products and toxins
  • The matured T cells remain inactive until they encounter their specific antigen
• Mature T lymphocytes


  Have many different types of surface receptor, each of which is complementary to a different antigen
• T lymphocytes in the immune response

  1. Antigen presentation: Macrophages engulf pathogens and present the pathogen antigens on their own cell surface membrane, they become antigen-presenting cells (APCs)

  2. Clonal selection: T cells with T cell receptors that are complementary to the specific pathogenic antigen bind to the APC, they are the clones that have been selected for replication

  3. Clonal expansion: Activated T cells divide by mitosis to produce clones
• T helper cells


  • Release chemical signaling molecules known as interleukins (a type of cytokines)
  • Interleukins causes phagocyte activity to increase
  • Interleukins is needed to activate B cells
• T killer cells


  • Patrol the body in search of antigen-presenting body cells
  • Attach to the foreign antigens on the cell surface membranes of infected cells and secrete toxic substances that kill the infected body cells, along with the pathogen inside
• Perforins


  • Secreted by T killer cells, punch a hole in the cell surface membrane of infected cells, allowing toxins to enter
• T regulatory cells


  • Down-regulate the host immune response by preventing T cells from attacking and killing uninfected host cells
  • Shut down the immune system once the body is cleared of the pathogen
• T memory cells


  • Remain in the blood, meaning that if the same antigen is encountered again the process of clonal selection will occur much more quickly
• Without checks the immune system can spiral out of control and cause serious damage to the host