Bach2 is a basic region-leucine zipper (bZip) transcription factor that forms heterodimers with small Maf oncoproteins and binds to target genes, thus repressing their expression. Bach2 is required for class switch recombination (CSR) and somatic hypermutation (SHM) of immunoglobulin genes in activated B cells. Bach2 represses the expression of Prdm1 encoding Blimp-1 repressor and thereby inhibits terminal differentiation of B cells to plasma cells. This causes a delay in the induction of Prdm1, thereby securing a time window for the expression of Aicda encoding activation-induced cytidine deaminase (AID) required for both CSR and SHM. Based on the characteristics of a gene regulatory network (GRN) involving Bach2 and Prdm1 and its dynamics, a 'delay-driven diversity' model was introduced to explain the responses of activated B cells. Bach2 is also required for the proper differentiation and function of peripheral T cells. In the absence of Bach2, CD4(+) T cells show increased differentiation to effector cells producing higher levels of Th2-related cytokines, such as IL-4 and IL-10, and a reduction in the generation of regulatory T cells. Bach2 represses many genes in T cells, including Prdm1, suggesting that the Bach2-Prdm1 pathway is also important in maintaining the homeostasis of T cells. Furthermore, Bach2 is essential for the function of alveolar macrophages. Therefore, Bach2 orchestrates both acquired and innate immunity at multiple points. Its connection with disease is also reviewed in this report.