Molecular architecture of an immunoglobulin G1 (IgG1) antibody. An IgG consists of two heavy chains (blue) and two light chains ( pink). (Left) The crystal structure of an antigen-binding fragment (Fab; Protein Data Bank identification number, 3NZ8). The Fab is composed of variable heavy (VH) and light (VL) domains as well as two constant domains (CH1 and CL). Each variable domain displays three binding loops (complementarity-determining regions, CDRs), which mediate antigen recognition. The CDRs in the VH domain are denoted as H1, H2, and H3 (blue); the CDRs in the VL domain are denoted as L1, L2, and L3 ( pink). (Right) The crystallizable fragment (Fc; Protein Data Bank identification number, 1E4K) contains two constant domains (CH2 and CH3) as well as glycans in the CH2 domain ( green). The Fc fragment mediates antibody effector function.