Cell mediated immunity

• • The body also has specific responses that react to specific antigens, providing long term immunity. This specific immune response depends on a type of white blood cell called a lymphocyte. Lymphocytes are produced by tem cells in the bone marrow.
• • B lymphocytes: MATURES in bone marrow, associated with HUMORAL IMMUNITY (involving antibodies that are present in body fluids or humour such as blood plasma) 
  • T lymphocytes: Matures in the thymus, associated with cell mediated immunity (immunity involving body cells)
• • Lymphocytes respond to an organism's own cells that have been infected by non self material from another material like a virus. These have different antigens on cell surface membranes from antigens on organisms' own cells.
• T- lymphocytes can distinguish between these invader cells from normal cells because

  • Phagocytes that have engulfed and hydrolyzed a pathogen present some of a pathogens antigens on their own cell surface membrane
  • Body cells invaded by a virus present some of the viral antigens on their own cell-surface membrane.
  • Transplanted cells from individuals of the same species have different antigens on their cell surface membrane
  • Cancer cells are different from normal body cells and present antigens on their cell surface membrane.
• Antigen presenting cells:

  Cells that display foreign antigens on their surface are called antigen - presenting cells because they can present antigens of other cells  on their own cell surface membrane.
• T lymphocytes will only respond to antigens that are presented on a body cell, this is a cell mediated immunity.

  1. Pathogens invade body cells or are taken in via phagocytosis.
  2. The phagocyte places antigens from the pathogen onto its cell surface membrane
  3. Receptors on a specific HELPER T cell fits exactly into these antigens
  4. The attachment activates the T cell to divide rapidly by mitosis and form a clone of genetically identical cells.
• Cloned T cells after cell mediated immunity:

   a) develop into memory cells that enable a rapid response to future infections by the same pathogens.

  b) stimulate phagocytes to engulf pathogens by phagocytosis

  c) stimulate B cells to divide and secrete their antibody

  d) activate cytotoxic T cells.
• How do cytotoxic T cells kill infected cells?

  • kill abnormal cells and body cells infected by pathogens, by producing a protein called perforin that makes holes in the cell surface membrane. These holes mean the cell membrane becomes free to all substances so the cell dies as a result.