The immunoglobulin hinge (interdomain) region

The hinge region is a flexible amino acid stretch in the central part of the heavy chains of the IgG and IgA immunoglobulin classes, which links these 2 chains by disulfide bonds. It is rich in cysteine and proline amino acids, extremely variable in amino acid sequence, and has no resemblance to any other immunoglobulin region. The hinges in these 2 classes are compared and contrasted. Such a distinct molecular structure does not exist around the inter-heavy disulfide bonds of the other Ig classes, but a portion of the IgM heavy chain has similar properties to the gamma hinge. This similarity suggests one hypothesis for the genetic origin of the hinge region; data supporting 2 other hypotheses are also presented.