Animal – Primary Non-specific Defences

Question 1

Skin:

Answer 1

Physical barrier blocking pathogens from entering body. Outer layer = epidermis (cells have keratin – hard). Also chemical barrier produces antimicrobial chemicals e.g. HCl that lowers pH, inhibiting pathogen growth. Lysozyme is an enzyme that is secreted in body fluids – catalyses breakdown of bacterial cell walls.

Question 2

Mucous membranes:

Answer 2

Air/food we take in may contain microorganisms, so airways, lungs and digestive system are at risk of infection and needs to remove pathogens before they reach thin exchange surfaces.
The epithelial layers contain goblet cells (mucus-secreting cells). Mucus lines the airways and traps pathogens in the air. Ciliated epithelial cells, cilia are tiny hair-like organelles waft the mucus up to the top of the trachea where mucus passes into the oesophagus and is swallowed. Most pathogens are killed by the stomach’s acidity – denatures pathogen’s enzymes. Found in the gut, genital areas, anus, ears and nose.

Question 3

Expulsive reflexes:

Answer 3

Areas prone to attack are also sensitive, they respond to irritation caused by presence of dust/dirt, microorganisms or the toxins they release. Reflexes include coughing, sneezing and vomiting, they expel air/fluid containing microorganisms causing the irritation.

Question 4

Blood clotting:

Answer 4

Abrasions/cuts damage the skin and open the body to infection. Clots prevent excess blood loss and make a temporary seal to prevent infection while repairing the skin. Blood clotting involves clotting factors released from platelets when they are exposed to damaged blood vessels. Platelets release thromboplastin (which triggers cascade of reactions that results in clot) and prostaglandins (makes smooth muscle of vessels contract, reducing blood flow). A mesh of fibrin fibres is formed from fibrinogen. Blood clotting is complex as clots shouldn’t form in vessels.

Question 5

Wound repair:

Answer 5

Once a clot has formed it dries out and forms a scab. The scab shrinks drawing skin together and forming a temporary seal, under which skin is repaired. Fibrous collagen is deposited under the scab, stem cells in the epidermis divide by mitosis to form new cells, which migrate to edges of the cut form new skin.

Question 6

Inflammation:

Answer 6

• Swelling and redness of tissue caused by infection, may feel hot and painful.
  The presence of microorganisms in the tissue is detected by mast cells, which release a cell signalling substance called histamine which causes:
• Vasodilation (increased blood flow – hot and brings more white blood cells).
• Capillary walls to become more permeable (leaky) so phagocytic white blood cells and some plasma
  proteins leave blood and enter tissue fluid, to engulf pathogens.
• This leads to increased tissue fluid production – swelling (which isolates pathogens).
• Excess tissue fluid is drains into the lymphatic system, where lymphocytes are stored leading to pathogens meeting lymphocytes and initiating specific immune responses.
• Stimulation of cells to release cytokines that promote phagocytosis and cause fever.

Question 7

Other primary defences:

Answer 7

• Eyes protected by antibodies and enzymes in tear fluid.
• Ear canal lined with wax that traps pathogens.

Question 8

Note:

Answer 8

Primary non-specific defences: Skin, mucous membranes and expulsive reflexes – prevent pathogen entry.

Secondary non-specific defences: clotting, inflammation, wound repair and phagocytosis.
(Tertiary) specific response: specific immune response below.