Schematic structure of a mAb. All immunoglobulins are composed of two identical light (L) chains and two identical heavy (H) chains, linked by disulphide bonds (black dashed bars). The heavy chains contain one variable domain (VH) and three or four constant domains (CH1, CH2, CH3 and CH4) depending on antibody isotype. By contrast, the light chains contain only one variable domain (VL) and a single constant domain (CL). Within the Fab region, at the end of the two arms of the Y-shaped molecule, the variable domain of a heavy chain pairs with the light chain variable domain to form the antigen-binding site. In more detail, within the matched V regions, three short polypeptide segments on the heavy chain and three on the light chain form the complementarity-determining regions (CDRs), which dictate the precise antigen-binding characteristics of the antibody. On the other end, the Fc domain, which includes the sites for interaction with the complement system and Fc receptors, mediates effector functions determining the fate of the bound antigen.