# Explain how the body distinguishes between 'self' and 'non-self' cells, and outline why this distinction is essential for an effective immune response.

> IB DP Biology Standard Level (2023 syllabus) — C3.2 Defence against disease · Explain · 4 marks

## Mark scheme (4 marks)

1. Every cell displays surface proteins (antigens / self-antigens / MHC markers) that identify it as belonging to the organism.
2. 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).
3. Antigens on pathogens / foreign cells / transplanted tissue are recognised as non-self, triggering an immune response (lymphocyte activation / antibody production / phagocytosis).
4. 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

- [Revision notes for IB DP Biology Standard Level (2023 syllabus)](https://www.gradenine.co.uk/learn)
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