Which body system is primarily responsible for defending against invading organisms?

The immune system is primarily responsible for defending against invading organisms, such as bacteria, viruses, and other pathogens. It comprises a complex network of cells, tissues, and organs that work together to recognize and eliminate these foreign invaders. The immune response involves various components, including white blood cells like lymphocytes and phagocytes, antibodies, and signaling molecules that help coordinate the body's defense mechanisms.

Unlike the circulatory system, which is mainly involved in transporting blood, nutrients, and oxygen throughout the body, or the nervous system, which regulates bodily functions and responses through nerve signals, the immune system specifically targets and neutralizes threats to maintain health. The respiratory system also does not play a direct role in immune defense; while it facilitates the exchange of gases and can be a route for pathogens to enter the body, it does not have the primary function of defending against infections. Thus, the immune system stands out as the key player in recognizing and combating infectious organisms, making it the correct answer in this scenario.