Defence against Infectious Diseases

Question 1

What are the main chemical and physical surface barriers against diseases?

Answer 1

• Removal of particles by cilia in nasopharynx
• Lysozyme in tears + other secretions
• Mucus lining in trachea
• Intact skin, fatty acids, commensals
• Rapid pH change in gut
• Commensals (normal flora)
• Vaginal acids (females)
• Flushing of urinary tracts

Question 2

How does the skin act as a surface barrier?

Answer 2

• Protects external structures when intact (outer body areas)
• Consists of a dry, thick + tough region composed predominantly of dead surface cells
• Contains biochemical defence agents (sebaceous glands secrete chemicals + enzymes which inhibit microbial growth on skin)
• Skin also secretes lactic acid + fatty acids to lower pH (skin pH roughly 5.6 - 6.4 depending on body region)

Question 3

How does the mucous membrane act as a surface barrier?

Answer 3

• Protects internal structures (i.e. externally accessible cavities + tubes: trachea, oesophagus or urethra)
• Consists of thin region of living surface cells that release fluids to wash away pathogens (mucus, saliva, tears, etc.)
• Contains biochemical defence agents (secretions contain lysozyme which can destroy cell walls and cause cell lysis
• Mucous membranes may ciliated to aid in the removal of pathogens (along with physical actions such as coughing/sneezing)