The lymphocytic series is the process of development and maturation of lymphocytes, a type of white blood cell. It primarily occurs in the lymphoid tissues, such as the bone marrow, thymus, and lymph nodes. The lymphocytic series includes the differentiation of precursor cells into B cells and T cells, which are important components of the immune system. B cells produce antibodies to fight against foreign substances, while T cells play various roles in immune responses, including the direct killing of infected cells.