Both B-cells and T-cells are derived from the __________.

B-cells and T-cells are derived from lymphocytes, which are a type of white blood cell essential for the immune response. Lymphocytes are categorized into several types, primarily B-cells and T-cells, both of which play crucial roles in the adaptive immune system. B-cells are involved in the production of antibodies, while T-cells are important for cell-mediated immunity, including helping to regulate the functions of other immune cells and directly killing infected host cells.

Plasma cells, which are mentioned in one of the choices, are actually differentiated forms of B-cells that produce antibodies once they have been activated. Monocytes and macrophages are different types of immune cells that play roles in the innate immune response, primarily involved in phagocytosis and the presentation of antigens to T-cells, rather than deriving from B-cells and T-cells. Thus, the correct answer highlights the foundational source of these critical immune cells in the body.