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

Antibodies contain a variable (hypervariable) region that folds into a specific tertiary structure forming the antigen‑binding site. This site is complementary in shape and charge to a particular epitope on a pathogen’s surface antigen, allowing binding through non‑covalent interactions.

Each antibody is Y‑shaped and has two identical antigen‑binding sites, so one antibody can bind two antigens. This bivalency enables cross‑linking of pathogens, leading to agglutination.

The constant (Fc) region of the antibody can bind to receptors on phagocytes, acting as an opsonin. Opsonisation together with agglutination prevents the pathogen from infecting host cells and facilitates its removal, thereby neutralising the pathogen.