While the biology of IgD begins to be better understood, the mechanism of expression of this phylogenetically old and highly conserved Ig class remains unknown. In B cells, IgD is expressed together with IgM as transmembrane receptor for antigen through alternative splicing of long primary VHDJH-Cμ-s-m-Cδ-s-m RNA, which also underpins the secreted form of IgD. IgD is also expressed through class switch DNA recombination (CSR), as initiated by AID-mediated double-strand DNA breaks (DSBs) in Sμ and σδ and resolution of such DSBs by a yet unknown alternative endjoining (A-EJ) mechanism. This synapses Sμ with σδ region DSB resected ends leading to insertion of extensive S-S junction microhomologies, unlike the Ku70/Ku86-dependent NHEJ which resolves DSB blunt ends in CSR to IgG, IgA and IgE with little or no microhomologies. We previously demonstrated a novel role of DNA annealing homologous recombination Rad52 protein in 'short-range' microhomology-mediated synapsis of intra-Sδ region DSBs. This led us to hypothesize that Rad52 is also involved in the short-range microhomology-mediated A-EJ recombination of Sμ with σδ. We found that induction of IgD CSR by T-dependent or T-independent stimuli downregulated Zfp318 (the suppressor of Cδ-s-m transcription termination), promoted Rad52 phosphorylation, recruitment of Rad52 to Sμ and σδ leading to Sμ-σδ recombination with extensive microhomologies, VHDJH-Cδs transcription and sustained IgD secretion. Rad52 ablation in mouse Rad52-/- B cells aborted IgD CSR in vitro and in vivo and dampened the specific IgD antibody response to OVA. Further, Rad52 knockdown in human B cells virtually abrogated IgD CSR. Finally, Rad52 phosphorylation was associated with high levels IgD CSR and anti-nuclear IgD autoantibodies in lupus-prone mice and lupus patients. Thus, Rad52 mediates CSR to IgD by synapsing Sμ-σδ resected DSB ends through microhomology-mediated A-EJ and in concert with Zfp318 modulation. This is a previously unrecognized, critical and dedicated role of Rad52 in mammalian DNA repair.