What are the two major classes of lymphocytes?

• A. B-cells and T-cells
• B. Natural killer cells and helper cells
• C. B-cells and plasma cells
• D. T-cells and macrophages

Answer: (A)

Lymphocytes are the white blood cells central to the adaptive immune response. The two main groups of lymphocytes are B cells and T cells. B cells become antibody-producing factories: after activation, they can differentiate into plasma cells that secrete antibodies, which circulate to tag pathogens and recruit other parts of the immune system. T cells mature in the thymus and carry out cell-mediated immunity. Helper T cells coordinate the response by activating B cells and other immune cells, while cytotoxic T cells directly kill infected or abnormal cells. Natural killer cells are also lymphocytes, but they belong to the innate side of immunity and aren’t counted with the primary B and T cell classes in this framework. Plasma cells are just mature B cells, not a separate class, and macrophages come from monocytes, not lymphocytes.