Which of the following is a type of antibody-mediated defense?

The selected answer, which is the production of antibodies by B cells, represents a key aspect of the humoral immune response, categorizing it as a type of antibody-mediated defense. In this process, B cells are activated upon encountering antigens, which leads to their differentiation into plasma cells that secrete antibodies. These antibodies specifically target and neutralize pathogens, such as bacteria and viruses, thereby assisting in their elimination from the body.

The ability of antibodies to bind to specific antigens facilitates several immune functions, including neutralization of toxins, opsonization (marking pathogens for destruction), and activation of the complement system, which can lead to the lysis of pathogens. This specificity and versatility make antibody-mediated defense a critical component of adaptive immunity.

In contrast, the other options pertain to different immune functions. Natural killer cell activity is a component of the innate immune system that targets and destroys infected or cancerous cells, while macrophage phagocytosis involves the engulfing and digestion of pathogens, also part of the innate immune response. Cytokine signaling refers to the communication between immune cells that helps regulate the immune response but does not directly pertain to antibody production.