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The primary role of B cells in active immunity is to produce antibodies that target and neutralize foreign molecules, such as pathogens or toxins. When B cells encounter an antigen (a molecule from a pathogen), they become activated and undergo a process of clonal expansion, leading to the formation of plasma cells. These plasma cells are specialized to produce large quantities of antibodies, which can bind to specific antigens, marking them for destruction by other immune cells or neutralizing their effects directly.

This antibody production is crucial for the adaptive immune response, as it enables the body to remember specific pathogens and mount a stronger response upon subsequent exposures. This memory response is a key feature of active immunity, distinguishing it from passive immunity, where antibodies are transferred from one individual to another without the recipient's immune system actively producing them.

The other options describe functions of different immune system components. For example, engulfing bacteria is primarily a function of phagocytes (such as macrophages), directly attacking infected cells is typically the role of cytotoxic T cells, and regulating the immune response is largely managed by helper T cells and regulatory T cells. Thus, the specific ability of B cells to produce antibodies is what underscores their critical role in active immunity.