Distinguish self and non-self antigens
Explain what an antigen is and distinguish self antigens from non-self antigens.
AS Level · Cambridge International Biology 9700, 2028–2030. Written practice is self-checked; human subject-teacher review and supervised practical evidence remain separate.
Before you begin
- Pathogens carry antigens, and specific receptors on lymphocytes can recognise complementary antigen shapes.
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Quick readiness check
What is an antigen?
What you will learn
- Define an antigen in the context of a specific immune response
- Distinguish self antigens from antigens recognised as foreign
Build the explanation
An antigen is a molecule or molecular structure that can be specifically recognised by receptors or antibodies and can trigger an immune response. Non-self antigens are recognised as foreign, for example on a pathogen. Self antigens are molecules associated with an individual's own cells and tissues. Lymphocyte receptors are specific: a receptor binds only to an antigen with a complementary shape. Recognition of self and non-self helps the immune system respond to pathogens while normally tolerating the body's own cells.
Apply it carefully
Do not define every antigen as a whole pathogen: antigens are structures associated with a pathogen or cell. A foreign substance can carry many different antigens. Self recognition is not perfect in every disease, and the syllabus-level definition does not require explaining autoimmune disorders. Use 'recognised as non-self' rather than implying that every foreign molecule necessarily causes disease.
Pause and explain
What is a non-self antigen in this context?
Worked example
A lymphocyte receptor binds a matching molecule on a bacterium. What is the molecule, and how is it classified for the host?
Show the worked solution
- The receptor recognises an antigen.
- On a pathogen, it is a non-self antigen.
Answer It is a non-self antigen associated with the bacterium; its shape is complementary to the lymphocyte receptor.
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Define antigen and give one example of a non-self antigen.
Give me a hint
State recognition and then give a pathogen-associated example.
Compare my reasoning
- An antigen is a molecule or structure recognised specifically by the immune system.
- A surface molecule on a pathogen can act as a non-self antigen.
An antigen is a molecule or structure specifically recognised by immune receptors or antibodies and capable of triggering a response. A surface antigen on a pathogen is a non-self antigen for the host.
Look for these in your work
- Recognition definition given
- Non-self example correct
A lymphocyte receptor does not bind an antigen with a different shape. Explain why this is useful.
Give me a hint
Connect shape complementarity to specificity.
Compare my reasoning
- Receptors have specific binding sites.
- Only a complementary antigen binds effectively, so the response is specific.
The lymphocyte receptor has a binding site with a particular shape. A complementary antigen can bind, giving specificity and helping target the response to the matching non-self antigen.
Look for these in your work
- Receptor specificity stated
- Complementary shape linked to response
A transplanted cell carries different surface antigens. Explain why a host immune system may recognise them as non-self without describing the transplant outcome.
Give me a hint
Focus on antigen recognition, not a clinical prediction.
Compare my reasoning
- Antigens are surface molecules recognised by immune receptors.
- Antigens different from the host's self pattern may be recognised as foreign.
The transplanted cell can display surface antigens that differ from the host's own antigens. Host lymphocyte receptors may recognise these as non-self and initiate a specific immune response; the actual clinical outcome depends on additional factors.
Look for these in your work
- Surface antigens identified
- Self/non-self distinction applied
- Outcome not overclaimed
Common mistakes
- Defining an antigen only as a pathogen
- Treating any foreign molecule as automatically harmful
- Claiming self/non-self recognition is infallible
What can you explain now?
What is an antigen?
Compare with the explanation
A molecule or structure specifically recognised by the immune system
Specific recognition depends on complementary receptor and antigen shapes.
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Make it stick
How does complementary receptor shape allow the immune system to distinguish a matching non-self antigen?
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