Vaccines are effective because the body "remembers" the exposure to the vaccine's component and can react swiftly to attack and destroy these components the next time they are encountered naturally in the environment. This occurs because lymphocytes called "B cells" are activated by certain antigens and, when combined with a chemical signal from T cells, divide rapidly to form two groups of many identical cells (clones). One group forms the active disease-fighting army of plasma cells and secretes antibodies into the blood. The second group remains stored in lymph nodes, ready to develop rapidly into more plasma cells at the next exposure to the same antigen.

What are these warrior cells, waiting in reserve in the lymph nodes, called?

A. cytotoxic cells
B. memory cells
C. macrophages
D. antibodies