12.5 Non-specific animal defences against pathogens

• Non-specific response

  • always present
  • activate quickly
  • defends against all pathogens in the same way
• physical barriers:

  • skin
  • skin flora
  • mucous membrane
  • lysozomes
• skin - covers the entire body to prevents entry of bacteria. Produces sebum, an oily substance that inhibits growth of pathogens
• skin flora - large population of naturally beneficial bacteria ofr surface space
• mucous membrane - ventilation of surfaces lined e.g tracges. Secretes sticky mucous which traps microorganisms and contains lysozymes which destroy bacteria
• lysozymes - in tears and urines, and the acid in your stomach. Destroys pathogens
• blood clotting:

  • if the skin is ruptured pathogens may enter the blood

  1. platelets rush to the site they release two substances; thromboplastin and serotonin - serotonin cause muscles to contract around the wound reducing the gap

  2. clot dries out forming a hard scab keeping pathogens out

  3. epidemical (skin) cells grow beneath the scab, collagen fibres reinforce the skin cells
• blood clotting:

  exposure of collagen fibres activate platelets

  platelets change shape, forming a temporary platelet plug. They produce thromboplastin

  thromboplastin activate a series of enzymes and convert prothrombin to thrombin

  vitamin K and calcium ions are needed

  thrombin catalyses the conversion of fibrinogen to fibrin

  when fibrin molecules are formed these create a mesh of fibrin forming a clot
• inflammation - a localised response to a pathogen or damage or irritants

  • pain
  • redness
  • heat
  • swelling
• inflammation:

  mast cells activated when skin is ruptured they release histamines and cytokines

  histamines - make blood vessels dilate, causing localised heat and redness also increase the permeability of the cell wall, causing tissue fluid to escape

  high temperatures prevent pathogens reproducing

  cytokines - attract white blood cells to deal with any pathogens. They dispose pathogens by phagocytosis
• oedema - histamines make blood vessels more leaky. blood plasma forced out (tissue fluid). tissue fluid causes oedema and pain
• fevers:

  • normal core temperature is 37 degrees
  • controlled by hypothalamus
  • during infection the presence of large numbers of large white blood cells cause hypothalamus, to reset and increase temperature
  • this is because pathogens can't reproduce as quickly with high temperatures and specific immune system can work faster
• non-specific response to pathogens:

  • physical and chemical barriers
  • phagocytosis
• phagocytosis - phagocytes are specialised white blood cells that engulf and destroy pathogens
• types of phagocytes:

  • neutrophils
  • macrophages
• neutrophils - rapidly engulf and destroy pathogens at the site of infection
• macrophages - engulf and digest pathogens but also present the pathogens antigens on the its cell surface to activate other cells in the immune system
• phagocytosis steps: in neutrophils

  1. pathogen releases chemical with attract phagocyte
  2. phagocyte recognises pathogen as 'non-self/foreign'. Phagocyte binds to pathogen
  3. phagocyte engulfs pathogen and encloses it in a phagosome
  4. phagosome combines with lysosome to form a phagolysosome
  5. lysozymes digest and destroy the pathogen
  6. broken down pathogen is moved out of phagocyte by exocytosis
• phagocytosis steps: in macrophages

  1. lysozymes digest and destroy the pathogen
  2. macrophage combines antigens from the pathogen cell surface membrane with special glycoproteins in the cytoplasm called MHC.
  3. MHC complex moves the pathogens antigens
  4. which then presents pathogens antigens on its cell surface to attract other cells in the immune system. phagocyte now known as an antigen-presenting cell (APC)
• phagocytes that have engulfed a pathogen produce cytokines they

  acts as cell signalling molecules, informing other phagocytes that the body is under attack and stimulating them to move to the site of infection. They also increase body temperature and stimulate the specific immune system
• opsonins - chemicals that binds to pathogens and 'tag' them so they can be more easily recognised by phagocytes. Phagocytes have receptors on their cell membranes that binds to common opsonins. This makes it easier for phagocytes to bind and destroy pathogens