F(aby2 composed of two lavers of polypeptide chains linked by a disulfide intrachain bridge in the centre of the domain.The domain.For the H chain.the v.8 and a chains consist of one variable (VH)and three constant (Cul to CH3) enghnCr.CSndEcha domains,an a fr units.are almost always pr sent.Simple or branched,the immunoglob udicd.The quence similarity among various domains sug sts that they are phylogenetically related,having evoved from a nd Fc of the in noglobulir is a sh. This proline- nonglobu porti on o olypeptid other This fe N-linked sugar antigen binding and effector functions.The cysteine -5-5-Disulfide bond Diagramof ins together. ween the Hand L chain Iso er in the numb t ( nt c ocation of interchain disulfide bonds and in the number of oligosaccharide moieties attached to the H chain.In of dditio nd onstant At the amino acid level,the variable region is comprised t and it is cap of binding and prec ating the three ne variab ilty which includ rontact the virtue of m and precipit ating the The Fab the antigen- mining or CDRs.Intersp ong the CDR are framework residues that represent approximately80% receptors of various effector cells. of the variable region ess varable and In humans,both k andchains of the Lchain consist of D pproximately amino acid residues By contras the izes P 446n dues for th the antigenic determinant(also called an epitope).Because 1 chai of the the u chain.This length variability is due essentially to the complexity of antigens each nolecule rent th ns med continu or linea Othe Each antibody chain is discontinuous or conformational epitopes,involve sites nd a variable region.In addition,comparison of antibody distributed in dif erent parts of the molecule,but brought amino acid sequences has revealed thre the existence appro rystals have revealed that the constant regions of the H amino a tr ur and L chains do not contribute to formation of the ENCYCLOPEDIA OF LIFE SCIENCES/62001 Naand precipitating the antigen. The Fab is the antigen￾binding constituent and the Fc fragment is the anchoring site for proteins of the complement system and for receptors of various effector cells. In humans, both k and l chains of the L chain consist of approximately 250 amino acid residues. By contrast, the different types of H chain vary considerably in length, from 446 amino acid residues for the g1 chain to 550 residues for the m chain. This length variability is due essentially to the presence of additional residues at the C-terminal end of the m and a chains. These extra amino acids participate in polymerization of immunoglobulins. Each antibody chain is composed of a constant and a variable region. In addition, comparison of antibody amino acid sequences has revealed the existence of homology regions, or domains, each of approximately 110 amino acids. Each domain is a globular structure composed of two layers of polypeptide chains linked by a disulfide intrachain bridge in the centre of the domain. The chains exhibit a b-strand conformation and form two antiparallel b sheets that pack together. An L chain is composed of one variable (VL) and one constant (CL) domain. For the H chain, the g, d and a chains consist of one variable (VH) and three constant (CH1 to CH3) domains, and the m and e chains exhibit an extra constant domain (CH4). Carbohydrate chains of variable shape and length, ranging from two sugar residues to a dozen or more units, are almost always present. Simple or branched, the carbohydrate chains normally attach to the Fc portion (the CH2 domain) of the immunoglobulin. This domain organization appears to be an important feature shared by immunoglobulins from all species studied. The sequence similarity among various domains suggests that they are phylogenetically related, having evolved from a single ancestral gene. Linking the Fab and Fc regions of the immunoglobulin is a short segment of the H chain between the CH1 and CH2 domains, known as the ‘hinge’, with no sequence similarity to any of the other domains. This proline- and cysteine￾rich, nonglobular portion of the polypeptide backbone allows segmental flexibility of the Fab arm and Fc region relative to one another. This flexibility is important for antigen binding and effector functions. The cysteine residues form interchain disulfide bonds linking the two H chains together. In addition to these interchain bridges, most immunoglobulins comprise a disulfide bridge be￾tween the H and L chain. Isotypes differ in the number and location of interchain disulfide bonds and in the number of oligosaccharide moieties attached to the H chain. In addition, the m and e chains contain an additional constant region domain that replaces the hinge region found in the g, d and a chains. At the amino acid level, the variable region is comprised of three regions of extreme variability which include residues that may contact the antigen by virtue of mutual complementarity. They are called complementarity-deter￾mining regions, or CDRs. Interspersed among the CDRs are framework residues that represent approximately 80% of the variable region and are less variable and more evolutionarily conserved. At the three-dimensional level, the three CDRs of each chain converge to form a combining site (also called a paratope) which recognizes the antigenic determinant (also called an epitope). Because of the complexity of macromolecular antigens, each molecule has many different epitopes. Some of them are composed of a single segment of the molecule and are termed continuous or linear epitopes. Others, called discontinuous or conformational epitopes, involve sites distributed in different parts of the molecule, but brought together in the three-dimensional structure. X-ray diffraction crystallographic studies of antibody crystals have revealed that the constant regions of the H and L chains do not contribute to formation of the S S Fab -S-S￾Fc CDRs S S -S-S￾CH1 VH CL VL N-linked sugar -S-S- Disulfide bond F(ab)’2 CH2 CH3 Figure 1 Diagram of a prototypic immunoglobulin (Ig)G monomer. Each rectangle represents an immunoglobulin domain. The complementarity￾determining regions (CDRs) are highlighted. The green bars depict the hinge region. The Fc fragment (for ‘fragment crystalline’) is produced by papain digestion of IgG and can be crystallized from a solution. It is the anchoring site for proteins of the complement system and for receptors of various effector cells. The Fab fragment (for ‘fragment antigen binding’) is produced by papain digestion of IgG. It possesses one combining site and can bind, but cannot precipitate, the antigen. The F(ab)’2 fragment is produced by pepsin digestion of IgG. Its molecular weight is double that of one Fab fragment, and it is capable of binding and precipitating the antigen. Antibodies 4 ENCYCLOPEDIA OF LIFE SCIENCES / & 2001 Nature Publishing Group / www.els.net