A 38-year-old man with chronic lymphocytic leukaemia (CLL) is treated with rituximab, a monoclonal antibody against CD20 on B lymphocytes. Within 48 hours, a marked reduction in circulating B cells is observed. Flow cytometry shows increased annexin V positivity in the remaining CLL cells. Which pathway of apoptosis is primarily responsible for rituximab-induced cell death in this patient?