IgE-producing B cells are always considered as ‘bad’ guys in allergic diseases, as they play a direct role in promoting inflammatory responses against inhaled or ingested allergens and contribute to the development of subsequent clinical symptoms. During the past decade, mouse studies have shown that B cells may also have a ‘better’ side, producing IL-10 and suppressing inflammatory responses; those cells are called regulatory B (Breg) cells. There is evidence now that impairment of Breg cell activity may play a role in the development of inflammatory diseases. New transgenic models and renewed interest have expanded our knowledge on different B cell phenotypes, signals to promote their activity and their function in health and disease. This review will highlight the current knowledge on disease-promoting or disease-protecting activities of B cells and discuss its potential for therapeutic applications in allergies.