Which option best identifies B cells?

• A. Involvement primarily in phagocytosis
• B. Location in thymus
• C. Ability to kill target cells without exposure
• D. Presence of surface antibody

Answer: (D)

B cells are best identified by the presence of surface antibodies, which act as their receptors for antigens. This membrane-bound immunoglobulin allows a B cell to recognize a specific antigen and become activated to produce antibodies as part of the humoral immune response. The other statements describe features of other immune cells: phagocytosis is mainly carried out by phagocytes like macrophages and neutrophils; the thymus is where T cells mature; and the ability to kill target cells without exposure is a trait of cytotoxic T cells (and NK cells). Thus, having surface antibody is the defining feature that identifies B cells.