The function and regulation of immunoglobulin D

Recent discoveries of IgD in ancient vertebrates suggest that IgD has been preserved in evolution from fish to human for important immunological functions. A non-canonical form of class switching from IgM to IgD occurs in the human upper respiratory mucosa to generate IgD-secreting B cells that bind respiratory bacteria and their products. In addition to enhancing mucosal immunity, IgD class-switched B cells enter the circulation to ‘arm’ basophils and other innate immune cells with secreted IgD. Although the nature of the IgD receptor remains elusive, cross-linking of IgD on basophils stimulates release of immunoactivating, proinflammatory and antimicrobial mediators. This pathway is dysregulated in autoinflammatory disorders such as hyper-IgD syndrome, indicating that IgD orchestrates an ancestral surveillance system at the interface between immunity and inflammation.

Research highlights
► IgD is expressed by B cells via alternative mRNA splicing and CSR.
► IgD CSR involves a cryptic σδ switch region and requires AID.
► IgD CSR occurs in upper respiratory mucosa B cells through TD and TI pathways.
► IgD stimulates basophil release of immunostimulating and antimicrobial factors.
► IgD CSR and production are dysregulated in autoinflammatory syndromes.