This review provides an analysis of the structural features of antibody molecules and their correlation with biological function. Antibodies are composed of three major fragments: two Fabs, which contain the light chain and the first two domains of the heavy chain, and an Fc, which contains the C-terminal constant domains of the two heavy chains. The Fabs are linked to the Fc by the hinge region, which varies in length and flexibility among different antibody classes and isotypes. The antigen-binding sites (paratopes) are located at the tips of the Fabs. The structure of antibodies is characterized by the Immunoglobulin Fold, a compact globular structure formed by a stable arrangement of hydrogen-bonded, anti-parallel β-strands, further stabilized by a disulfide bond between the two layers. The variable domains (VL and VH) and the constant domains (CL and CH1) are similar in structure and are superposable across species. The hypervariable regions (CDRs) are responsible for antigen recognition and are highly variable among different antibodies. The Fv region, formed by the association of VL and VH, is a compact module and is related by a pseudo-dyad. The constant domains (CL and CH1) also form a compact module and are related by a pseudo-dyad. The variable and constant modules are connected by a short segment of polypeptide chain called the switch. The angle between the VL-VH and CL-CH1 pseudo-dyads is called the "elbow bend" of the Fab and varies due to the flexibility of the switch. The CDRs are loops situated at the N-terminal tip of the Fab and form a continuous surface of approximately 2800 Ų. The CDR surface is characterized by depressions and protrusions and may contain a deep pocket or a cleft. The CDR surface is primarily involved in antigen binding and is usually equated with the combining site of the antibody (the paratope). The CDRs are disposed such that the N-terminal part of CDR1-L and the C-terminal parts of CDR2-L and CDR2-H are farther from the center of the CDR surface, while CDR1-H, CDR3-H, CDR3-L, the C-terminal part of CDR1-L, and the N-terminal parts of CDR2-L and CDR2-H are closer to the center. The interactions between the CDRs and the framework residues are significant and contribute to the variation seen in the quaternary association of the variable domains. The CDR and framework residues involved in the VL-VH interaction in various antigen-binding regions are given in tables. Intradomain framework-CDR interactions can influence the conformation of the CDRs. The framework residues that interact with the CDRs are mostly from FR2-L and FR4-L in VL and FR2-H in VH. The frameworkThis review provides an analysis of the structural features of antibody molecules and their correlation with biological function. Antibodies are composed of three major fragments: two Fabs, which contain the light chain and the first two domains of the heavy chain, and an Fc, which contains the C-terminal constant domains of the two heavy chains. The Fabs are linked to the Fc by the hinge region, which varies in length and flexibility among different antibody classes and isotypes. The antigen-binding sites (paratopes) are located at the tips of the Fabs. The structure of antibodies is characterized by the Immunoglobulin Fold, a compact globular structure formed by a stable arrangement of hydrogen-bonded, anti-parallel β-strands, further stabilized by a disulfide bond between the two layers. The variable domains (VL and VH) and the constant domains (CL and CH1) are similar in structure and are superposable across species. The hypervariable regions (CDRs) are responsible for antigen recognition and are highly variable among different antibodies. The Fv region, formed by the association of VL and VH, is a compact module and is related by a pseudo-dyad. The constant domains (CL and CH1) also form a compact module and are related by a pseudo-dyad. The variable and constant modules are connected by a short segment of polypeptide chain called the switch. The angle between the VL-VH and CL-CH1 pseudo-dyads is called the "elbow bend" of the Fab and varies due to the flexibility of the switch. The CDRs are loops situated at the N-terminal tip of the Fab and form a continuous surface of approximately 2800 Ų. The CDR surface is characterized by depressions and protrusions and may contain a deep pocket or a cleft. The CDR surface is primarily involved in antigen binding and is usually equated with the combining site of the antibody (the paratope). The CDRs are disposed such that the N-terminal part of CDR1-L and the C-terminal parts of CDR2-L and CDR2-H are farther from the center of the CDR surface, while CDR1-H, CDR3-H, CDR3-L, the C-terminal part of CDR1-L, and the N-terminal parts of CDR2-L and CDR2-H are closer to the center. The interactions between the CDRs and the framework residues are significant and contribute to the variation seen in the quaternary association of the variable domains. The CDR and framework residues involved in the VL-VH interaction in various antigen-binding regions are given in tables. Intradomain framework-CDR interactions can influence the conformation of the CDRs. The framework residues that interact with the CDRs are mostly from FR2-L and FR4-L in VL and FR2-H in VH. The framework