Which set lists all three major lymphocyte classes that circulate in blood?

• A. T cells and B cells
• B. T cells, B cells, NK cells
• C. NK cells
• D. B cells and NK cells

Answer: (B)

Three major lymphocyte classes circulate in blood: T cells, B cells, and natural killer cells. T cells coordinate cell-mediated immunity and mature in the thymus; B cells produce antibodies and drive humoral immunity; NK cells provide rapid, nonspecific cytotoxic responses against virus-infected or transformed cells. The set that lists all three together is the correct one because it fully represents the major circulating lymphocytes. Sets that omit one class miss a key component of the lymphocyte population, making them incomplete. So this option is the best because it includes T cells, B cells, and NK cells.