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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

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

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.