Explain how the body distinguishes between 'self' and 'non-self' cells, and outline why this distinction is essential for an effective immune response.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (4 marks)
Every cell displays surface proteins (MHC I/II) that act as self‑antigens. During lymphocyte maturation, cells that bind strongly to these self‑antigens are deleted or anergised, giving self‑tolerance. When a pathogen or foreign cell presents a different antigen, lymphocytes recognise it as non‑self, become activated, and produce antibodies or phagocytose the invader. If the system fails to recognise self, it attacks its own tissues, causing autoimmune disease – showing why the self/non‑self distinction is essential for a correct immune response.
Examiner tips
- Use the terms ‘self‑antigen’, ‘MHC’, ‘clonal deletion’, ‘self‑tolerance’, ‘non‑self’, ‘lymphocyte activation’, ‘autoimmune disease’.
- Show the sequence: surface proteins → self‑tolerance → recognition of non‑self → immune response.
- Mention the consequence of failure (autoimmunity).
Common mistakes
- Confusing self‑antigens with foreign antigens; not naming MHC or clonal deletion; omitting the link to autoimmune disease.
Mark scheme (4 marks)
- Every cell displays surface proteins (antigens / self-antigens / MHC markers) that identify it as belonging to the organism.
- Lymphocytes (during maturation) are exposed to self-antigens so that any lymphocytes that react strongly to self are eliminated or suppressed (clonal deletion / self-tolerance).
- Antigens on pathogens / foreign cells / transplanted tissue are recognised as non-self, triggering an immune response (lymphocyte activation / antibody production / phagocytosis).
- Failure to distinguish self from non-self results in autoimmune disease (immune system attacks body's own cells / tissues), illustrating why the distinction is essential.
Related
- All IB DP Biology Standard Level (2023 syllabus) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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