12.5 NON-SPECIFIC ANIMAL DEFENCES AGAINST PATHOGENS

Question 1

what are the bodies barriers to the entry of pathogens?

Answer 1

• skin
• tracts
• lysosymes
• expulsive reflexes

Question 2

what are the skins defences?

Answer 2

• has a skin flora of healthy microorganisms that outcompete pathogens for space on the body surface
  -produces sebum, oily substance that inhibits the growth of pathogens

Question 3

what are tracts, and how do they defend the body from pathogens?

Answer 3

• include airways of gas exchange system
• lined by mucus membranes that secrete sticky mucus
• this traps microorganisms and contains lysosymes which destroy bacterial and fungal cell walls
• mucus also contains phagocytes, which remove remaining pathogens

Question 4

what are lysosymes?

Answer 4

• are in tears and urine and stomach acid

Question 5

what are expulsive reflexes and how do they help defend the body?

Answer 5

• coughs and sneezes eject pathogen-laden mucus from the gas exchange system
• vomiting and diarrhoea expel the contents of the gut along with any infecting pathogens

Question 6

what happens if you cut yourself?

Answer 6

• skin is breached and pathogens can enter the body
• blood clots rapidly to seal the wound
• when platelets come into contact with collagen in skin or the wall of the damaged blood vessel, they adhere and begin secreting several substances

Question 7

what are the most important substances secreted by platelets during blood clotting?

Answer 7

• thromboplastin (enzyme that triggers a cascade of reactions resulting in the formation of a blood clot/ thrombus)
• serotonin (makes smooth muscle in the walls of the blood vessels contract, so they narrow and reduce the supply of blood to the area)

Question 8

what are the stages of wound repair?

Answer 8

• clot dries out, forming a hard, tough scab that keeps pathogens out
• epidermal cells below the scab start to grow, sealing the wound permanently, while damaged blood vessels regrow
• collagen fibres are deposited to give the new tissue strength
• once the new epidermis reaches normal thickness, the scab sloughs off and the wound is healed

Question 9

what is the inflammatory response?

Answer 9

• localised response to pathogens (or damage/ irritants) resulting in inflammation at the site of a wound
• characterised by pain, heat, redness and swelling of tissue

Question 10

what are activated in damaged tissues and what do they release (during an inflammatory response)?

Answer 10

• mast cells
• release chemicals called histamines and cytokines

Question 11

what do histamines do in an inflammatory response?

Answer 11

• make blood vessels dilate, causing localised heat and redness
• this raised temperature helps prevent pathogens reproducing
• make blood vessel walls more leaky so blood plasma is forced out, once forced out of the blood it is known as tissue fluid
• tissue fluid causes swelling (oedema) and pain

Question 12

what do cytokines do in an inflammatory response?

Answer 12

• attract white blood cells (phagocytes) to the site
• they dispose of pathogens by phagocytosis

Question 13

what happens if an infection is widespread?

Answer 13

• inflammatory response can cause a whole body rash

Question 14

what are triggered in the secondary lines of defence (if pathogens get into the body)?

Answer 14

• fevers
• phagocytosis

Question 15

what is a fever?

Answer 15

• normal body temperature of around 37*C is maintained by the hypothalamus in your brain
• when a pathogen invades your body, cytokines stimulate your hypothalamus to reset the thermostat and your temperature goes up

Question 16

why is having a fever a useful adaptation?

Answer 16

• most pathogens reproduce best at or below 37*C, higher temperatures inhibit pathogen reproduction
• the specific immune system works faster at higher temperatures

Question 17

what are phagocytes?

Answer 17

• specialised white blood cells that engulf and destroy pathogens
• two main types are neutrophils and macrophages
• build up at the site of infections

Question 18

what are the stages of phagocytosis?

Answer 18

1. pathogens produce chemicals that attract pathogens
2. phagocytes recognise non-human proteins on the pathogen. this is a response not to a specific type of pathogen, but simply a cell or organism that is non-self
3. phagocyte engulfs pathogen and encloses it in a vacuole called a phagosome
4. phagosome combines with a lysosome to form a phagolysosome
5. enzymes from the lysosome digest and destroy the pathogen

Question 19

what happens when a macrophage has digested a pathogen?

Answer 19

• it combines antigens from the surface of the pathogen membrane with special glycoproteins in the cytoplasm
• this is called the Major Histocompatibility Complex (MHC)

Question 20

what does the MHC do?

Answer 20

• moves the pathogen antigens to the macrophage’s own surface membrane, becoming an antigen-presenting cell (APC)
• these antigens now stimulate other cells involved in the specific immune system response

Question 21

how are cytokines formed and how are they helpful?

Answer 21

• produced by phagocytes that have engulfed a pathogen
• act as cell signalling molecules, informing other phagocytes that the body is under attack and stimulating them to move to the site of infection or inflammation
• can also increase body temperature and stimulate specific immune system

Question 22

what are opsonins?

Answer 22

• chemicals that bind to pathogens and ‘tag’ them so they can be more easily recognised by phagocytes
• phagocytes have receptors on their cell membranes that bind to common opsonins, and the phagocyte then engulfs the pathogen