Exam Tips: Non-Specific Human Defences

State the barrier AND explain the mechanism: "Skin acts as a physical barrier" is one mark. "Skin acts as a physical barrier, preventing pathogens from entering underlying tissues, and produces sebum which creates an acidic pH that inhibits bacterial growth" is two or three marks.

Phagocytosis sequence must be complete: Recognise the pathogen as foreign using surface receptors → attach → cell membrane surrounds pathogen forming a phagosome → phagosome fuses with lysosome → lysosomal enzymes digest the pathogen → waste products expelled. All six steps required for full marks on an extended question.

Mucus and cilia work as a pair: Never describe one without the other. Mucus traps the pathogens; cilia move the mucus. Together they form the "ciliary escalator." If smoking destroys cilia, mucus cannot be swept away, increasing infection risk — a favourite exam application question.

Non-specific vs specific immunity distinction: Non-specific responds the same way to ANY pathogen; does not require previous exposure; does not produce memory. Specific (adaptive) immunity is tailored to one specific antigen, is slower on first exposure but faster on re-exposure, and creates memory cells. This distinction is tested frequently.

Required practical — use area not diameter for comparisons: The zone of inhibition is circular. Area = pi x r squared. This gives a more meaningful comparison than diameter alone. Higher-tier questions may ask you to calculate and compare areas from given diameter measurements.

Link to smoking for application questions: Smoking damages cilia in the bronchi, meaning mucus cannot be swept away. Bacteria and viruses accumulate in the airways. The smoker's "morning cough" is an attempt to clear mucus that cilia can no longer remove. This is a classic application of first-line defence theory.