Definition of antigen; the effect of antigen variability on disease and disease prevention
An antigen is a molecule on a cell's surface that the immune system can recognise as foreign. When antigens change shape, the immune system can no longer recognise them, making diseases harder to prevent.
Real World
Influenza viruses undergo antigenic shift — a dramatic change in surface protein shape — which caused the 2009 H1N1 swine flu pandemic because existing vaccines offered no cross-protection.
Exam Focus
Distinguish antigenic variation from mutation in your answer; examiners expect you to explain why existing antibodies/memory cells no longer bind the new antigen shape.
Evaluation Scaffold
A four-step framework for high-quality evaluation. Use this for 'assess', 'evaluate', and 'to what extent' questions.
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