A patient is given a blood transfusion using blood from a donor whose antigens do not match their own. Explain why the patient's immune system attacks the transfused blood cells, and describe what happens during this immune response.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
When a patient receives a blood transfusion, the donor's blood cells must be carefully matched to the patient's blood type. If mismatched blood is given, the patient's immune system mounts a response against the donated cells.
Model answer (5 marks)
The donor’s red cells display antigens that are not present on the patient’s own cells, so they are recognised as foreign. The patient’s immune system therefore mounts a response. B‑lymphocytes that recognise the foreign antigens are activated and produce antibodies that are specific to those antigens. The antibodies bind to the antigens on the transfused cells, forming antigen–antibody complexes. These complexes cause the donor cells to be destroyed – they may clump together and be removed by the spleen or be lysed, leading to haemolysis and damage to the patient.
Examiner tips
- Use the word ‘antigens’ and ‘antibodies’ – they are key terms. Show the sequence: foreign antigens → B‑cell activation → antibody production → binding → cell destruction. Keep it concise to fit 5 marks.
Common mistakes
- Confusing T‑cell with B‑cell role; forgetting that antibodies are produced by B‑cells. Writing ‘immune system attacks’ without explaining the antibody step.
Mark scheme (5 marks)
- The donor's blood cells carry different/foreign antigens on their surface
- The antigens are recognised as foreign / non-self, triggering an immune response
- B-lymphocytes produce antibodies specific to the foreign antigens
- Antibodies bind to the foreign antigens on the donor blood cells
- The donor blood cells are destroyed / clump together / are broken down, harming the patient
Key terms in this question
Related
- All Edexcel A-Level Biology A: Salters-Nuffield (9BN0) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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