6.3 Defense against Infectious Diseases

Question 1

surface barriers

Answer 1

first line of defense - incl intact skin and mucous membrane

skin:
- protects external structures when intact
- consists of a dry, tough region composed mostly of dead surface cells
- contains biochemical defenses (eg. sebaceous glands which secrete enzymes and chemicals to inhibit microbial growth)
- secretes lactic acid and fatty acids to lower its pH (~6.5 - 6.4, depending on region)

mucous membrane:
- protects internal structures (ie. externally accessible structures - esophagus, trachea, urethra)
- consists of thin layer of living cells which release fluids to wash away pathogens (mucus, saliva, tears)
- contains biochemical defense agents (secretions contain lysozymes which can destroy cell wall and cause cell lysis)
- may be ciliated to remove pathogens (together with actions such as sneezing and coughing)

Question 2

clotting - blood clots

Answer 2

• prevent blood loss from the body and limit pathogenic access to the body where skin is broken

key components:
platelets and insoluble fibrin threads
- platelets: undergo structural change when activated => sticky plug at damaged region (primary haemostasis)
- insoluble fibrin threads: fibrin strands form insoluble fibrin mesh which trap blood cells at the damaged region (secondary haemostasis)

Question 3

clotting - coagulation cascade (ie. secondary haemostasis)

Answer 3

CC is stimulated by clotting factors released by damaged cells (extrinsic pathway) and platelets (intrinsic pathway)

1. clotting factors cause platelets to become sticky and adhere to the damaged region to form a plug
2. clotting factors initiate vasoconstriction at site of damage to reduce blood flow through the damaged region
3. clotting factors trigger the conversion of inactive prothrombin to activated enzyme thrombin
4. Thrombin catalyzes the conversion of soluble fibrinogen into insoluble fibrin
5. fibrin forms a temporary mesh around the platelet plug => traps blood => temporary clot (thrombus)
6. when damaged region is completely repaired, an enzyme (plasmin) is activated to dissolve the clot

Question 4

clotting - coronary thrombosis

Answer 4

• formation of a clot within coronary arteries

cholesterol deposition develops in the coronary arteries => reduces diameter of lumen
restricted blood flow in the artery => increased pressure in the arteries => damages arterial wall
damaged region is repaired with fibrous tissue => reduces elasticity of arterial wall
smooth lining of wall progressively degraded => lesions (atherosclerotic plaques) form
plaque ruptures => clotting is triggered => thrombus => restricts blood flow
thrombus dislodged => embolus => cause blockage in smaller arterioles => myocardial infraction