B cells are primarily responsible for which function?

• A. Kill infected cells directly
• B. Produce antibodies that circulate in body fluids
• C. Phagocytose pathogens
• D. Secrete interferon

Answer: (B)

B cells mediate humoral immunity by producing antibodies that circulate in body fluids. When a B cell encounters its specific antigen and receives help from a helper T cell, it becomes a plasma cell that secretes large amounts of antibodies. These antibodies travel through the blood and extracellular spaces to bind pathogens or toxins, neutralizing them directly or tagging them for attack by other immune components, such as phagocytes or the complement system. This extracellular defense is the defining function of B cells.

Direct killing of infected cells is the job of cytotoxic T cells, not B cells. Phagocytosis is carried out by macrophages and neutrophils. Interferon is a cytokine produced by various cells to establish antiviral states, not a primary output of B cells.