12.5 - Non-specific animal defences against pathogens

Question 1

1 Non-specific defences - keeping pathogens out

Answer 1

• The skin covers the body and prevents the entry of pathogens. It has a skin flora of healthy microorganisms that outcompete pathogens for space on the body surface. The skin also produces sebum, on oily that inhibits the growth of pathogens
• Many of the body tracts, including the airways of the gas exchange system, are lined by mucous membranes that secrete sticky mucus. This traps microorganisms and contains lysosomes, which destroy bacterial and fungal cell walls. Mucus also contains phagocytes, which remove remaining pathogens.
• Lysosomes in tears and urine, and the acid in the stomach, also help to prevent pathogens getting into our bodies
• Expulsive reflexes: Coughs and sneezes eject pathogen-laden mucus from the gas exchange system, while vomiting and diarrhoea expel the contents of the gut along with infective pathogens

Question 2

1 Blood clotting and wound repair

Answer 2

• If you cut yourself, the skin is breached and pathogens can enter the body.
• The blood clots rapidly to seal the wound.
• When platelets come into contact with collagen in the skin or the wall of the damaged blood vessel, they adhere and begin secreting several substances. The most important are:
  1) thromboplastin - an enzyme that triggers a cascade of reactions resulting in the formation of a blood clot (or thrombus)
  2) seratonin - makes the smooth muscle in the walls of the blood vessels contract, so they narrow and reduce the supply of blood to the area.
• The clot dries out, forming a hard, tough scab that keeps pathogens out.
• This is the first stage of wound repair.
• Epidermal cells below the scab start to grow, sealing the wound permanently, while damaged blood vessels grow. 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 3

1 Inflammatory response

Answer 3

• Localised response to pathogens (or damage to irritants) resulting in inflammation at the site of a wound. Inflammation is characterised by pain, heat, redness, and swelling of tissue.
• Mast cells are activated in damaged tissue and release chemicals called histamines and cytokines
  1) Histamines make blood vessels dilate, causing localised heat and redness. The raised temperature helps prevent pathogens reproducing
  2) Histamines 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
  3) Cytokines attract white blood cells (phagocytes) to the site. They dispose of pathogens by phagocytosis
• If an infection is widespread, the inflammatory response can cause a whole-body rash

Question 4

Fevers

Answer 4

• Normal body temperature of 37’C is maintained by the hypothalamus
• When a pathogen invades your body, cytokines stimulate your hypothalamus to reset the thermostat and your temperature goes up.
• This is useful because:
  1) most pathogens reproduce best at or below 37’C. Higher temperatures inhibit pathogen reproduction
  2) The specific immune system works faster at higher temperatures.

Question 5

Phagocytosis

Answer 5

• Specialised white cells that engulf and destroy pathogens.
• Two main types of pathogens: neutrophils and macrophages
• Phagocytes build up at the site of an infection and attack pathogens
• Sometimes you can see pus in a spot, cut, or wound. Pus consists of dead neutrophils and pathogens
• The stages of phagocytosis are:
  1) Pathogens produce chemicals that attract phagocytes
  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) The phagocyte engulfs the pathogen and encloses it in a vacuole called a phagosome
  4) The phagosome combines with a lysosome to form a phagolysosome
  5) Enzymes from the lysosome digest and destroy the pathogen
• It usually takes a human neutrophil under 10 minutes to engulf and destroy bacterium.
• Macrophages take longer but they undergo a complex process. When a macrophage has digested a pathogen, it combines antigens from the pathogen surface membrane with special glycoproteins in the cytoplasm called the major histocompatibility complex (MHC). The MHC complex moves these 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 6

Counting blood cells

Answer 6

• Blood smears: made by spreading a single drop of blood very thinly across a slide
• They are often stained to show up the nuclei of the lymphocytes, making them easier to identify.
• Identifying the numbers of different types of lymphocytes in a blood smear indicates if . non-specific or specific immune response is taking place.

Question 7

Helpful chemicals

Answer 7

• Phagocytes that have engulfed a pathogen produce chemicals called cytokines
• Cytokins act as cell-signalling molecules, informing other phgocytes that the body is under attack and stimulating them to move to the site of infection or inflammation.
• Cytokines can also increase body temperature and stimulate the specific immune system
• Opsonins are 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, different opsonins, but antibodies such as immunoglobulin G (igG) and immunoglobulin m (IgM) have the strongest effect.