The T Lymphocyte Response (AQA A Level Biology): Revision Note

Lymphocytes

• Lymphocytes are another type of white blood cell that

  • play an important part in the specific immune response

  • are smaller than phagocytes

  • have a large nucleus that fills most of the cell

  • are produced in the bone marrow before birth

• There are two types of lymphocytes (with different modes of action)

  • T-lymphocytes (T cells) target and destroy abnormal or infected body cells; this is known as the cellular response

  • B-lymphocytes (B cells) produce antibodies that target antigens in body fluids; this is known as the humoral response

The T-Lymphocyte (cellular) response

Antigen-presenting cells

• An antigen-presenting cell is one of the host's cells that has been invaded by a pathogen and is displaying the antigen on its cell surface membrane

  • These antigens are displayed on major histocompatibility complexes (MHCs)

• Antigen-presenting cells activate specific T cells that have complementary receptors to the antigen

Activation of helper T cells

• Helper T cells are activated when they encounter (and bind to) their specific antigen on an antigen-presenting cell e.g. a macrophage

• These activated helper T cells divide by mitosis to form clones

• Helper T cells release cytokines, which stimulate

  • maturation of B-lymphocytes into antibody-secreting plasma cells

  • production of memory B and T cells

  • an enhanced rate of phagocytosis

  • activation of cytotoxic T cells (T killer cells)

Cytotoxic T cells

• T killer cells patrol the body for infected cells displaying foreign antigens

• They bind to these antigens and release toxic substances to destroy the infected cells and pathogens

• Perforins create holes in the membrane, allowing toxins to enter and kill the cell