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C3.2.7—Antigens

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

Syllabus
First assessment 2025
Objective
C3.2.7
Level
HL

Exam analysis

Chance of appearing1%of analysed past papers
Latest appearanceMay 2014
Most common paperPaper1
Typical marks1

Common command terms

  • Identify

Scoring notes

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

Recent exam appearances

May 2014Paper1 ["HL"] · TZ119[ 1 ]C3.2.7—Antigens
Practice this objective

Coverage 2014–2014 · Updated 16 Jul 2026

An antigen is the feature an immune receptor recognises

An antigen is a recognition molecule that can bind a specific antibody or lymphocyte receptor and trigger antibody production.

Most antigens are glycoproteins or other proteins and are located on the outer surface of a pathogen, where immune receptors can encounter them.

Antigen = molecular target, not the whole pathogen. Complementary receptor binding selects an antigen-specific response; different surface antigens select different lymphocyte clones.

Erythrocyte surface antigens from an incompatible blood group can stimulate antibody binding and production after transfusion, causing dangerous cell agglutination or destruction.

An antigen is not automatically an entire foreign organism. It is a recognizable molecule or molecular region, commonly a surface protein or glycoprotein.

Antigens exam focus

Assessment in practice

1 marks
How it is assessed

This objective is assessed through multiple choice, commonly using Identify.

Command terms

Identify

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: 1]

What is a characteristic of antigens?

A

They recognize foreign substances

B

They are produced in bone marrow

C

They cause disease in humans

D

They stimulate the production of antibodies

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

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