Which of the following defines the role of antibodies in the immune system?

Antibodies play a crucial role in the immune system primarily by binding to and neutralizing pathogens such as bacteria and viruses. This binding is essential because it can block the pathogen's ability to infect host cells, effectively neutralizing their harmful effects. Additionally, the binding of antibodies to pathogens signals other components of the immune system, facilitating their recognition and destruction. This process not only helps to eliminate the infection but also aids in the formation of memory cells, which provide immunity against future infections by the same pathogen.

The other options do not accurately define the role of antibodies. Stimulating immune cells refers more to the action of cytokines and other signaling molecules rather than antibodies themselves. Transporting oxygen is the primary function of hemoglobin in red blood cells. Producing toxins is associated with certain pathogens or immune cells but is not a function of antibodies. Hence, the accurate portrayal of antibodies' function is their ability to bind to and neutralize pathogens.