The First Example of an Isoallotype of Human IgG Located on the Fd Fragment.

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Abstract
An isoallotype of IgG is described that is an isotype on heavy chains of IgG3 and IgG4 proteins and an allotype for IgG1 proteins, in which it is associated with G1m(f). The isoallotypic determinant is located on Fd. Binding of light chains (independent of their type) is required for its antigenic expression, since isolated heavy chains are not reactive in inhibition tests. It is suggested that the coding for the amino acid sequence related to this isoallotype was present on the ancestral gene before the duplication of subclasses. The presence in many non-human primate species proves that it is strongly conserved in evolution.