11.2.1—Antibody molecular structure and functions

An antibody is a Y-shaped globular glycoprotein produced by B lymphocytes, including plasma cells. Its two heavy and two light polypeptide chains are joined by disulfide bonds; variable regions at the tips form antigen-binding sites.

• Variable region → specificity: the binding site is complementary to a region of an antigen called an epitope, so an antibody binds its matching antigen and forms an antigen–antibody complex.
• Hinge → flexibility: movement at the hinge helps the binding sites approach antigens at different angles.
• Constant region → effector action: after binding, the antibody can block a virus entering cells or neutralise a toxin; it can also make bacteria easier for phagocytes to recognise (opsonisation), clump pathogens together (agglutination), or contribute to lysis, including complement-associated lysis.

The same recognition logic links binding to different outcomes: specific antigen attachment blocks or neutralises a target directly, or marks/organises pathogens so phagocytes can engulf them; complement-associated action can contribute to pathogen lysis.

Variable regions determine which antigen is bound; the constant region helps determine what happens after binding. Antibodies do not digest pathogens themselves. The primary-response production sequence belongs to 4631, while hybridoma production, monoclonal uses, immunity types and vaccines belong to 4634–4638.