Antigens and Cell Recognition
What antigens are, how the immune system tells self from non-self, and why antigen variability matters for disease and vaccines. Includes the 4-mark exam question, a worked model answer and the examiner trap on confusing antigens with antibodies.
Chapters (8)
Exam question [4 marks]
Influenza viruses often change the proteins on their surface. Explain what is meant by an antigen, and why a vaccine against influenza made one year may not protect against influenza the next year.
Try it on paper first, then check the mark scheme below. The question is original and was written for this lesson.
Mark scheme
| Mark | Mark point | Also accept | Do not accept |
|---|---|---|---|
| 1 | An antigen is a molecule (usually a protein) on the surface of a cell / virus that triggers an immune response | "protein on the surface that causes an immune response", "foreign molecule that stimulates the immune system" | "an antigen is an antibody"; "a substance that kills pathogens"; "a pathogen" |
| 2 | The immune system recognises it as foreign / non-self because it differs from the body's own (self) antigens | "not recognised as part of the body", "different shape from own cells' molecules" | "the body recognises it as harmful" with no reference to difference from self |
| 3 | Antigen variability: mutation changes the shape of the virus's surface antigens | "antigens change / mutate", "new antigen shape", "different strain", "antigenic drift / shift" | "the virus changes" with no reference to antigens; "the virus evolves into a new disease" |
| 4 | The immune response / protection produced by the first vaccine no longer matches (is not specific to) the new antigens, so there is no protection | "antibodies / memory cells from the vaccine do not fit the new antigens", "not recognised, so no response", "a new vaccine is needed" only if linked to the old response not matching | "the person becomes immune to the vaccine"; "the vaccine stops working" with no reference to antigen shape |
Mark 1 is the definition; a candidate who calls antigens antibodies loses it and risks mark 4 as well. Mark 2 needs the idea of difference from self or non-self; "foreign" alone is credited once only (in mark 1). Marks 3 and 4 are linked: change of antigen shape, then the consequence for the earlier response. Antibodies and memory cells are taught in later lessons; here only "the earlier immune response does not match" is needed.
Model answer (4/4)
An antigen is a molecule, usually a protein, on the surface of a cell or virus that triggers an immune response. The immune system recognises it as foreign, or non-self, because it is different from the body's own molecules. Influenza shows antigen variability: mutation changes the shape of its surface proteins. The immune response produced after the earlier vaccine therefore does not match the new antigens, so it gives no protection and a new vaccine is needed.
Examiner trap
- Saying antigens are antibodies
- Saying antigens are only on pathogens
- Not linking antigen variability to vaccine failure
Key terms
- antigen
- pathogen
- self
- non-self
- antigen variability
- toxin
- abnormal cell
Lesson notes
A written version of the video, chapter by chapter, for revision. Times link to the video.
0:26Surface proteins identify cells
Every cell has molecules on its surface, and many of them are proteins. Each type of cell has its own specific set, a bit like a name badge. These surface molecules let the immune system identify what a cell is, and whether it belongs in your body. Different cell types have different molecules, so each one has its own identity.
0:48Self and non-self
Your own cells carry your own badges. The immune system recognises them as self, and leaves them alone. A cell with badges it does not recognise is non-self, and it triggers an immune response.
1:01What the immune system recognises
So what can the immune system spot? Four things. Pathogens, which are microorganisms that cause disease. Cells from another individual of the same species, for example a transplanted organ from another person. Abnormal body cells, such as cancer cells.
And toxins, which are poisonous substances.
1:22Definition of an antigen
Here is the definition to learn. An antigen is a molecule, usually a protein, that triggers an immune response. It sits on the surface of a cell or virus, or floats free like a toxin. An antigen is not an antibody. Antigens trigger. Antibodies are made in response. And antigens are not only on pathogens. Your own cells have them too, you just do not react to them.
1:50Antigen variability and vaccines
Now the flu question. Some pathogens show antigen variability. Their antigens change over time. This happens mainly because of mutation, which changes the shape of the surface protein. A vaccine causes an immune response to one particular antigen shape, before you ever meet the real virus. If the virus now carries a different shape, the response made after the vaccine is not specific to it.
The earlier immune response does not match the new antigen, so you are not protected. That is why flu vaccines are updated, and why people need a new one. So antigen variability makes some diseases hard to prevent, because the target keeps moving.
3:08Examiner trap and recap
The trap. Never call an antigen an antibody. And do not say antigens are only on pathogens. Recap. Antigens are surface molecules that identify a cell as self or non-self. When pathogen antigens change, the earlier immune response no longer matches.
Next, we follow the first cells that respond: phagocytes, and how they eat a pathogen.
Watch first
This lesson builds on:
- Protein Structure: Primary to Quaternary · AQA 7402 · 3.1.4.1
Keep going
More from 3.2 Cells
- Phagocytosis: How Phagocytes Work · AQA 7402 · 3.2.4
- T Lymphocytes: The Cellular Immune Response · AQA 7402 · 3.2.4
- B Cells, Clonal Selection and Antibodies · AQA 7402 · 3.2.4
- Primary and Secondary Immune Response · AQA 7402 · 3.2.4
Independent revision resource, not affiliated with or endorsed by AQA. The exam question is original and was written for this lesson; it is not an AQA question. Spotted an error? Email info@medservatory.com with the lesson code AB-041.