Which of the following best describes the difference in the way B cells and cytotoxic T cells deal with invaders? a. B cells confer active immunity; T cells confer passive immunity. b. B cells send out antibodies to attack; certain T cells can do the attacking themselves. c. T cells handle the primary immune response; B cells handle the secondary response. d. B cells are responsible for the cell-mediated immune response; T cells are responsible for the humoral immune response.

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1. Identify the roles of B cells and T cells in the immune system. B cells are primarily responsible for the humoral immune response, which involves the production of antibodies that circulate in the blood and lymph. These antibodies bind to antigens on pathogens to neutralize them or mark them for destruction.
2. Understand the function of cytotoxic T cells, a type of T cell. Cytotoxic T cells are involved in the cell-mediated immune response, where they directly attack and kill infected cells. They recognize infected cells through their T-cell receptors which bind to specific antigens presented on the surface of these cells.
3. Analyze the options given in the question to find which one correctly describes the roles of B cells and T cells. Option a suggests a distinction between active and passive immunity which is not specific to B cells and T cells. Option c incorrectly assigns primary and secondary responses to T cells and B cells, respectively.
4. Review option b, which states that B cells send out antibodies to attack, and certain T cells can do the attacking themselves. This aligns with the known functions of B cells producing antibodies and cytotoxic T cells directly attacking infected cells.
5. Conclude that the correct answer must accurately reflect the roles of B cells and T cells in the immune system based on their involvement in humoral and cell-mediated immunity, respectively.