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C3.2.2—Skin and mucous membranes

Skin and mucous membranes explains how a specific barrier, pathogen interaction, immune mechanism or treatment response contributes to protection against infectious disease.

Syllabus
First assessment 2025
Objective
C3.2.2
Level
HL

Exam analysis

Chance of appearing4%of analysed past papers
Latest appearanceNovember 2025
Most common paperPaper2
Typical marks1–3

Common command terms

  • State
  • Describe

Scoring notes

Common mistake
Naming symptoms without linking them to the causative pathogen or transmission route.

Recent exam appearances

November 2025Paper2 ["HL"] · TZ32(a)[ 2 ]C3.2.2—Skin and mucous membranes
May 2025Paper1B ["HL"] · TZ13(e)[ 2 ]C3.2.2—Skin and mucous membranes
May 2024Paper2 ["HL"] · TZ28(b)[ 3 ]C3.2.2—Skin and mucous membranes
November 2020Paper3 ["HL"] · TZ021(b)[ 1 ]C3.2.2—Skin and mucous membranes
Practice this objective

Coverage 2020–2025 · Updated 16 Jul 2026

Skin and mucous membranes block entry

Skin and mucous membranes block entry.

Keratinised skin forms a physical barrier, while mucus traps particles and cilia move them away. Secretions such as lysozyme, tears and stomach acid add chemical protection.

barrier; trapped or killed agent; route out; limitation if the barrier is breached.

Mucus in an airway traps microbes and cilia move the mucus toward the throat, reducing access to lung tissue.

These are first-line defences, not antigen-specific memory responses.

Skin and mucous membranes

Assessment in practice

1–3 marks
How it is assessed

This objective is assessed through structured response, commonly using State / Describe.

Command terms

State / Describe

What earns marks

Mark schemes repeatedly reward named details, correct sequence and clear distinctions between similar processes.

Watch for

Naming symptoms without linking them to the causative pathogen or transmission route.

Representative question

Question 1

[Maximum number: 3]

Describe how disease-causing bacteria are prevented from entering the body.

Defence Against Disease

  • Skin, mucus, cilia, lysozyme and clotting form primary barriers against pathogens.
  • Innate immunity is rapid and broad: phagocytes recognize, engulf and digest pathogens. Adaptive immunity is antigen-specific and forms memory.
  • Helper T-cells coordinate responses; activated B-cells undergo clonal selection, producing antibody-secreting plasma cells and memory cells. A second exposure therefore triggers a faster, stronger response.
  • HIV infects CD4 helper T-cells; their loss weakens immune coordination and can lead to AIDS.
  • Antibiotics target bacterial processes, not viruses. Antibiotic exposure selects resistant variants, which can spread by reproduction or plasmid transfer.
  • Vaccination creates active artificial immunity; high population immunity can indirectly protect susceptible people.
  • Evaluate disease and vaccine claims using reliable sources, trends, controlled comparisons, incidence and efficacy—not raw totals alone.

Concept essentials

  • Skin and mucous membranes has to be linked to the exact pathogen, cell, molecule or population process involved.
  • The biological effect of skin and mucous membranes depends on the sequence from trigger to protective outcome.
  • Clear distinctions within skin and mucous membranes prevent confusion with neighbouring immune responses.
  • Evidence for skin and mucous membranes is strongest when mechanism and consequence are explained together.