DNA acrobats of the Ig class switch.

Small resting B lymphocytes all start out producing IgM Abs. Upon encountering Ag, the cells become activated and make a switch from IgM to other Ig classes. This class switch serves to distribute a particular V region to different Ig C regions. Each C region mediates a specialized effector function, and so, through switching, an organism can guide its Abs to various sites. Creating the new H chain requires loop-out and deletion of DNA between switch regions. These DNA acrobatics require transcription of the switch regions, presumably so that necessary factors can gain access to the DNA. These requisite switching factors include activation-induced cytidine deaminase and components of general DNA repair, including base excision repair, mismatch repair, and double-strand break repair. Despite much recent progress, not all important factors have been discovered, especially those that may guide recombination to a particular subclass.