The origin and development (ontogeny) of the lymphocyte system, from its earliest stages to the two major populations of mature lymphocytes: the thymus-dependent or T lymphocytes, and the thymus-independent or B lymphocytes. The T lymphocytes carry out those aspects of function which are called cell-mediated immunity, including graft rejection, elimination of tumor cells, and delayed hypersensitivity. B cells are responsible for humoral or antibody-mediated immunity.