B cells are cells that regulate inflammation and immune responses in the immune system. B cells are typically characterized by their ability to produce antibodies, secondary antigen presenting cells, and various immune regulatory cytokines. Regulatory B cells (Bregs) are a subgroup of B cells that play an important role in many conditions, including infections, allergies, autoimmune diseases, transplantation and cancer. Breg cells perform their immune regulation functions through contact with cytokines they secrete and other immune cells (Wang et al. 2020). In this study, the phenotype of Breg cells, their functions in the immune system, their role in autoimmune and allergic diseases and cancer will be discussed.