Immunoglobulin molecules of the class G (antibody molecules) consist of two heavy chains (50,000 dalton molecular weight) and two light chains (25,000 dalton). The overall shape is a Y with the arms formed by the light chains and the N-terminal half of the heavy chains in tight association. The stem is formed by the C-terminal halves of the heavy chains. The heavy and the light chains fold into globular domains of molecular weights of 12,000 dalton. There are four domains of the heavy chain and two of the light chain. All these domains show a similar fold, consisting of two B-sheets but display considerable differences in detail. The N-terminal variable domains of heavy and light chains and specifically the hypervariable polypeptide segments of the domains, located at the tips of the Y, constitute the antigen and hapten binding site. The nature of the amino acid residues of the hypervariable loops determines the shape and the specificity of the antibody. All domains pair tightly laterally, except the CH2 domains of the heavy chain. This domain has carbohydrate bound which prevents lateral association. Longitudinal interaction between the domains is loose and allows flexibility in the arrangement. Flexibility is probably of significance for antibody function. Arm (Fab) and stem (Fc) parts are linked by the hinge peptide which contains a segment with a unique conformation of two parallel poly-proline helices. Antigen binding triggers effector functions of antibodies. Antigen binding is at the tips of the Y-shaped antibody, but effector functions are displayed by the stem part. It is an open question whether conformational changes of the antibody molecule play a significant role in the trigger mechanism.