Answer
All lymphocytes are produced in the red bone marrow. T-lymphocytes (T-cells) go to the thymus for maturation and differentiation, but B lymphocytes (B-cells) mature in the red bone marrow. Therefore, "a" is correct--B-cells have not passed through the thymus.
B-cells respond to antigens specific for their receptors (BCR); they synthesize antibodies ( plasma B-cells), and they are responsible for antibody-mediated immunity.
Work Step by Step
Both B-cells and T-cells are parts of the adaptive immune system: they respond to antigens( non-self substances or cells) that get into the body. When naive B-cells first encounter their specific antigen the B-cell receptor (BCR) binds with the specific antigen and the B-cell undergoes clonal selection. A clone is group of similar cells. In this case the cells of this clone includes plasma cells and memory cells. Plasma cells immediately secrete antibodies to the antigen, and the antigen-antibody reaction destroys the antigenic substance/organism-- plasma B-cells are responsible for antibody-mediated immunity. After the antigen-antibody reaction is over, plasma cells undergo apoptosis (programmed cell death). Memory B-cells remain in the body ; they are able to react quickly if the same antigenic substance (chemical type) that initiated their differentiation/selection enters the body again.
