# Explain how the structure of an antibody enables it to neutralise a specific pathogen.

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

## Mark scheme (4 marks)

1. Antibodies have a variable region (hypervariable region) with a specific tertiary structure / shape that forms the antigen-binding site
2. The antigen-binding site is complementary to a specific epitope on the pathogen's surface antigen, allowing binding by non-covalent interactions
3. Each antibody molecule has two identical antigen-binding sites (Y-shaped / bivalent structure), enabling cross-linking / agglutination of pathogens into clumps
4. Agglutination / opsonisation (via the constant/Fc region) facilitates phagocytosis, preventing the pathogen from infecting host cells / neutralising the pathogen

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