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C3.2.3—Blood clotting

Blood clotting explains how a specific barrier, pathogen interaction, immune mechanism or treatment response contributes to protection against infectious disease.

Syllabus
First assessment 2025
Objective
C3.2.3
Level
SL

Exam analysis

Chance of appearing7%of analysed past papers
Latest appearanceNovember 2024
Most common paperPaper1
Typical marks1–3

Common command terms

  • Identify
  • Describe
  • State
  • Explain
  • Outline

Scoring notes

Common mistake
Reversing fibrinogen and fibrin in the clotting sequence.

Recent exam appearances

November 2024Paper1 ["SL"] · TZ228[ 1 ]C3.2.3—Blood clotting
May 2024Paper2 ["SL"] · TZ15(c)[ 3 ]C3.2.3—Blood clotting
May 2024Paper1 ["SL"] · TZ227[ 1 ]C3.2.3—Blood clotting
May 2023Paper1 ["SL"] · TZ228[ 1 ]C3.2.3—Blood clotting
May 2022Paper2 ["SL"] · TZ26(c)[ 7 ]C3.2.3—Blood clotting
Practice this objective

Coverage 2017–2024 · Updated 16 Jul 2026

Clotting seals a damaged blood vessel

Blood clotting rapidly seals a cut in the skin, limiting blood loss and pathogen entry.

Platelets at damaged tissue release clotting factors that start a cascade. The cascade produces thrombin, which rapidly converts soluble fibrinogen into insoluble fibrin strands.

Cut → platelet activation and clotting-factor release → cascade → thrombin → fibrinogen converted to fibrin → fibrin mesh traps erythrocytes → clot seals wound.

At a skin cut, the fibrin mesh stabilizes the platelet plug and traps red blood cells, forming a clot that can dry into a protective scab.

Thrombin converts fibrinogen to fibrin; it is not the mesh itself. Further clotting-factor details are outside this objective.

Blood clotting

Assessment in practice

1–2 marks
How it is assessed

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

Command terms

Identify / Describe / State / Explain / Outline

What earns marks

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

Watch for

Reversing fibrinogen and fibrin in the clotting sequence.

Representative question

Question 1

[Maximum number: 7]

Explain how blood clotting occurs and the consequence for a person who has hemophilia.

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

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