T lymphocytes and cell mediated immunity

• Antigen = usually a protein but can also be a polysaccharide or glycoprotein and is part of the cell surface membrane or cell wall of invading cells
• Self-antigen = recognised as your own and does not normally stimulate an immune response
• Foreign antigen = stimulates an immune response and triggers the production of antibodies
• Antibodies = proteins synthesised by plasma cells, have a specific tertiary structure and a region that is complementary to the shape of a foreign antigen
• Examples of antigen presenting cells = phagocytes, body cells invaded by virus, cancer cells, cells from other organisms of the same species
• Cell mediated immunity

  • pathogens ingested by phagocytes
  • phagocyte "presents" foreign antigen on cell surface membrane
  • receptors on some T cells fit exactly onto foreign antigens
  • T cells are activated to divide rapidly by mitosis and form clones
• 4 types of T cells = cytotoxic, helper, memory, suppressor
• Memory T cells

  • respond to future invasion of pathogen with same antigen
  • retain memory as they have cell surface receptors for specific antigens
  • can enable rapid response to repeated infection - secondary response
• Cytotoxic T cells

  • kills cells that have the specific foreign antigens on their surface
  • secrete protein called perforin which makes holes in cell membrane of antigen-presenting cells to kill them
• Helper T cells secrete chemicals called cytokines that stimulate

  • cytotoxic cells to destroy pathogens/kill infected cells
  • phagocytes to carry out more phagocytosis
  • stimulate B lymphocytes to divide and become plasma cells
• Suppressor T cells close down the immune system activity when the pathogen has been destroyed
• Each pathogen contains a variety of different antigens on its surface, each one stimulates the production of a different type of T cell