Structure of other immunoglobulins in relation to IgG

As shown in Fig. 10, IgG, IgA and IgD have a similar arrangement of three constant domains and a hinge region. For IgM and IgE the hinge is replaced by an extra domain. With the involvement of tail pieces, IgM and IgA are able to form polymeric molecules. IgM is found almost exclusively as a pentamer whereas IgA is found principally as a monomer or dimer. 

Light (L) chain A or k chain L = VL + CL domains Heavy (H) chain 

Intradomain disulphides link Cys-367 (or Cys-144) to Cys-425 (or Cys-200) with Trp-381 (or Trp-158) nearby. CM4 and C 3 have an extra tailpiece cysteine involved in polymer formation. 

Comparison of human immunoglobulin domains of the IgG CM2 type and IgM and IgE CH2 domains 

IgG Ch2 type refers to non-paircd domains with interposed carbohydrate chains attached to Asn-297. Cs2 lacks this carbohydrate but has a chain at Asn-258 which could function similarly, as discussed in the text. C 2 and Cf2 are included separately for the sake of the completeness. They probably more closely resemble IgG CH1 or C,(3 than Cn2. A dot indicates no residue at the position corresponding to the numbered human IgGl residue. N indicates N-linked carbohydrate and S O-linked carbohydrate. 

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