Which system is adversely affected by toxicants like benzene and dioxins, making us more susceptible to infectious diseases?

The immune system is the correct choice because toxicants such as benzene and dioxins can significantly weaken its functionality. These substances can impair the body's ability to fight off infections by disrupting immune responses, reducing the production and effectiveness of immune cells, and altering the balance of cytokines, which are critical for communication within the immune system. As a result, exposure to these toxicants can lead to a higher susceptibility to infectious diseases, making it harder for the body to defend itself against pathogens.

In contrast, while the respiratory, circulatory, and nervous systems can also be affected by toxic substances, the most direct and critical impact regarding susceptibility to infections specifically relates to the immune system. Therefore, the immune system is uniquely positioned as the primary defense mechanism in the body that is compromised by these toxic exposures, highlighting its vital role in maintaining health and preventing illness.