6.9—Antigens, antibodies and immune response

Antigens are surface markers that distinguish self from non-self. An antibody has a variable region whose binding site is complementary to a particular antigen, so the antigen–antibody complex is specific.

Specific binding can neutralise toxins, block pathogen attachment or agglutinate pathogens for easier removal. Phagocytes can present pathogen antigens to activate lymphocytes.

An antibody against a bacterial surface antigen can bind that bacterium but not a different pathogen with another epitope; the variable region, not the constant region, provides the matching site.

Antibodies do not recognise every pathogen equally, and binding alone is not the same as killing. Keep antigen recognition, antibody action and later cell-mediated responses distinct.