Final answer:
B cells are responsible for humoral immunity, whereas cytotoxic T cells are necessary for cellular immunity.
Step-by-step explanation:
B cells are responsible for humoral immunity, which involves the production of antibodies that fight pathogens in extracellular spaces. On the other hand, cytotoxic T cells are necessary for cellular immunity, specifically targeting and eliminating intracellular pathogens. T cells also play a central role in coordinating the overall adaptive immune response.