Antibody (Immunoglobulin) Structure

An antibody, or immunoglobulin, is a Y-shaped protein produced by plasma cells to bind and neutralise a specific antigen. Each antibody molecule is built of four polypeptide chains: two identical, longer heavy (H) chains and two identical, shorter light (L) chains, together denoted H2L2, held together by disulphide bonds.

The two arms of the Y each end in a region called the Fab (fragment antigen-binding) end — this is where the antibody actually grips its target, and its sequence varies from antibody to antibody so that different antibodies can recognise different antigens. The base of the Y is the Fc (fragment crystallisable) end, whose sequence is constant within a given class of antibody and which interacts with immune cells (for example, phagocytes) and with the complement system. …