An antigen is a marker on a pathogen, and an antibody is a protein the body makes to bind to it. The response runs in five steps, from recognition to memory cells.
This lesson follows non-specific versus specific defence. Here you follow the specific response step by step.
What are the five steps?
- A pathogen with an antigen enters the body.
- A lymphocyte with a matching receptor recognises the antigen.
- That lymphocyte multiplies into many identical cells.
- These cells make antibodies with a shape that fits the antigen.
- Some cells stay as memory cells, ready for a later infection.
Worked example: a text diagram
This example uses an invented bacterium called “Type X”.
Type X enters -> antigen on its surface
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Lymphocyte with matching receptor recognises it
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Lymphocyte multiplies into many identical cells
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Cells make antibodies that fit the Type X antigen
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Antibodies bind to Type X -> phagocytes engulf it
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Some cells remain as memory cellsQuestion: Explain how the body responds to Type X.
Full answer: “Type X carries an antigen on its surface. A lymphocyte with a matching receptor recognises it and multiplies, and the new cells make antibodies with a shape that fits the antigen. The antibodies bind to Type X and mark it for phagocytes, and memory cells remain for a faster response next time.”
The mistake that loses marks
The common slip is to swap the terms, as in “The antibody on the bacterium is recognised by the antigen.” The roles are the wrong way round.
| Term | Belongs to | Made by |
|---|---|---|
| Antigen | The pathogen | The pathogen |
| Antibody | The body | Lymphocytes |
The fix is one rule: the body makes the antibody, and the pathogen carries the antigen.
A routine for any antigen-antibody question
- Name the antigen and say it is on the pathogen.
- State that a matching lymphocyte recognises it.
- Say the lymphocyte multiplies and makes antibodies that fit.
- Add what the antibodies do and that memory cells remain.
The biological data interpretation trainer has timelines that show antibody levels over time.
Check yourself
Invented case: a second infection by the same Type X occurs months later. Explain why the response is faster.
Answer
Memory cells made during the first infection remain in the body and recognise the Type X antigen quickly. They multiply and make antibodies faster and in larger amounts, so the pathogen is removed sooner.
What to study next
Compare how protection is gained in comparing active and passive immunity. For the stage before antibodies, revisit non-specific versus specific defence.
If you want a teacher to go through the diagram with you, see online one-to-one Biology tuition.