Internal sequence homologies provided the first indication that the L and H polypeptide chains fold into discrete and independent domains, each consisting of about 110 amino acids and containing a 60 residue dilsulfide loop. This has been confirmed by electron microscopy and X-ray diffraction analysis. The domains are numbered from the N-terminus: Vr and C|. (k or \) in the case of the light chain, and VM and C71, C72 and C73 in the case of the heavy chain. The V or variable domains show extensive sequence variation associated with their antigen-binding speci ficity, while the constant domains exhibit only subclass (isotypic) and allotypic variation in amino acid sequence. There are four subclasses of the heavy (y) chains and there are allotypic variations within these subclasses (Table 1).

The mid-section of the 7 chain is characterized by a region of extended polypeptide chain known as the hinge region. Its purpose seems to be to extend the molecule and to facilitate segmental flexibility of the parts of the molecule. Such flexibility must be of critical importance in enabling the two antigen-bind-ing sites which are formed by the N-terminal variable

Table 1 Physical, chemical and biological properties of the four human immunoglobulin G subclasses

Property lgG1




H chain type Mol. wt

Hinge amino acid number

Number of interchain disulfide bonds

Plasma half-life (days)

Serum concentration (mg ml 1)

Complement fixation

Placental transfer

Fc receptor Fc,RI

Fc receptor Fc,RII

Fc receptor Fc^RIII

Hinge sequences

Upper: starting at residue 216


Lower: (N-terminus of C,2) 231 Allotypic markers

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