12.5

• Animals keep pathogens out using: Skin, mucous membranes, lysozymes, and blood clotting
• The skin is a physical barrier and has a skin flora of healthy microorganisms that outcompete pathogens
• The skin produces sebum (oily substance that inhibits growth of pathogens)
• Body tracts are lined with mucous membranes which secrete sticky mucus that traps microorganisms and contains lysozymes & phagocytes
• Lysozymes in tears and urine help to keep pathogens out
• Stomach acid destroys microorganisms
• Blood clotting is where platelets adhere to the collagen in the skin or wall of damaged blood vessel and secret substances
• Thromboplastin is an enzyme that triggers a cascade of reactions resulting in the formation of a blood clot
• Serotonin causes the smooth muscle in blood vessels to contract to reduce the blood supply to the damaged area
• Scabbing: the clot dries out to form a tough scab, fibrin mesh forms around platelets, epidermal cells below grow to seal wound permanently, collagen fibres give new tissue strength, scab sloughs off once epidermis has reached normal thickness
• The Inflammatory response is a localised response resulting in inflammation
• Inflammatory response: Mast cells are activated in damaged tissue and release histamines & cytokines
• Histamines cause dilation of blood vessels which causes localised heat & redness (raised temperature prevents pathogens reproducing), and make vessel walls more leaky so tissue fluid is forced out which causes swelling
• Cytokines attract phagocytes to the site so pathogens are disposed of by phagocytosis
• Fevers are caused when cytokines stimulate the hypothalamus to reset the thermostat which causes the temerature to rise - Temps above 37 degrees inhibit pathogen reproduction
• Phagocytosis: pathogens produce chemicals that attract phagocytes, phagocytes recognise non-human proteins on pathogen, phagocyte engulfs pathogen and encloses it in a phagosome (vacuole), phagosome combines with lysosome to form phagolysosome, enzymes from lysosome digest & destroy pathogen
• Pus consists of dead neutrophils & pathogens
• Neutrophils take under 10 minutes to engulf and destroy a bacterium
• Macrophages take longer to destroy pathogens as they undergo a more complex process
• Major Histocompatibility Complex (MHC) is made up of special glycoproteins found in the cytoplasm
• After digestion, antigens from the pathogen surface membrane are combined with MHC which transfers the antigens to the macrophages surface membrane to become and antigen-presenting cell - the antigens can now stimulate other cells in the specific immune response