The separate development of functionally intertwined lineages of lymphocytes known as B cells and T cells is now recognized as a fundamental organizing principle of the adaptive immune system in all vertebrates. Immunologists strive to define the different sublineages of the clonally diverse B cells and T cells, how they interact with each other and how they interact with innate lymphoid cells and other elements of the innate immune system to counter infections, cancer and the development of autoimmune and inflammatory diseases. On the 50th anniversary of the recognition of B cells as a discrete cell lineage, this Timeline article recounts some of the milestones marking the development of the concept that B cells are a functionally and developmentally distinct arm of the adaptive immune system.