4.1.7 The T Lymphocyte Response

• Lymphocytes and antibodies provide the third line of defence against pathogens
  • Unlike the first and second lines of defence, the third line is specific
  • Specific immune responses are slower but more effective than non-specific immune responses

• Lymphocytes are
  • 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

• There are two types of lymphocytes (with different modes of action)
  • T-lymphocytes (T cells)
    • Lymphocytes that mature in the thymus gland

  • B-lymphocytes (B cells)
    • Lymphocytes that mature in the bone marrow

Maturation of T-lymphocytes

• Immature T-lymphocytes originate in the bone marrow
• They move to the thymus gland in the chest, which is where they mature
• During the process of maturation T lymphocytes (T cells) gain specific cell surface receptors called T cell receptors (TCRs)
  • These receptors have a similar structure to antibodies and are each complementary to a different antigen
  • A small number of T cells have the same TCRs, these genetically identical cells are called clones
    • T cells within each clone differentiate into different types of T cell: T helper cells, T killer cells and T regulator 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

• In order to play their role in the immune response T cells need to be activated and increase in number; this process is described below
• Antigen presentation
  • Macrophages engulf pathogens and present the pathogen antigens on their own cell surface membrane
  • They become antigen-presenting cells (APCs)
• 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
  • Binding to the complementary antigens causes the T cell to be activated
• Clonal expansion
  • Activated T cells divide by mitosis to produce clones
• There are now many T cells in the blood, all of which have specific roles
  • T helper cells
    • These cells release chemical signalling molecules known as interleukins (a type of cytokines)
    • Interleukins causes phagocyte activity to increase
    • Interleukins is needed to activate B cells
  • T killer cells
    • T killer cells patrol the body in search of antigen-presenting body cells
      • T killer 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
    • Without checks the immune system can spiral out of control and cause serious damage to the host
    • T regulator cells down-regulate the host immune response by
      • Preventing T cells from attacking and killing uninfected host cells
      • Shutting down the immune system once the body is cleared of the pathogen
  • T memory cells
    • 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

Activated T cells divide by mitosis to produce clones. Cloned T helper cells produce chemicals that activate B cells while cloned T killer cells destroy infected body cells.