Which substance is primarily responsible for activating B cells?

B cells are a crucial component of the adaptive immune response and require activation to differentiate into plasma cells that produce antibodies. The primary substance responsible for activating B cells is cytokines released by helper T cells. When a helper T cell recognizes an antigen presented by an antigen-presenting cell (APC), it releases cytokines that provide essential signals for B cell activation and proliferation. These cytokines include interleukin-4 (IL-4) and interleukin-5 (IL-5), which stimulate B cell growth, differentiation, and the class-switching required for producing different types of antibodies.

While antibodies do play a significant role in the immune response by identifying and neutralizing pathogens, they are produced by B cells after activation, not responsible for activating B cells themselves. Antigens from pathogens are crucial for the initial recognition and binding by B cells but do not provide the necessary proliferation signals. Complement proteins, although involved in promoting inflammation and pathogen clearance, do not directly activate B cells; instead, they enhance the immune response in other ways. Thus, cytokines from helper T cells are essential for the specific activation of B cells, making this the correct choice.