The T Lymphocyte Response (OCR A Level Biology)

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Specific Immune Response: T Lymphocytes

  • 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 and T killer 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

_The maturation of T-lymphocytes, downloadable AS & A Level Biology revision notes

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 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

The function of T-lymphocytes during an immune responseActivated 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.

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Lára

Author: Lára

Expertise: Biology Lead

Lára graduated from Oxford University in Biological Sciences and has now been a science tutor working in the UK for several years. Lára has a particular interest in the area of infectious disease and epidemiology, and enjoys creating original educational materials that develop confidence and facilitate learning.