White blood cells are of primary importance in which of the following systems?

The respiratory system primarily focuses on the exchange of gases—oxygen and carbon dioxide—between the body and the environment. While some immune functions may occur in the respiratory tract, such as the activity of certain immune cells in lung tissue, white blood cells are not primarily associated with the mechanics or functions of respiration.

The excretory system is responsible for the removal of waste products from the body, including the regulation of water and electrolyte balance. Although immune cells can be present in the urinary tract and other excretory components, they are not primarily involved in the functions of this system, which centers around filtration and waste elimination.

The nervous system is tasked with transmitting signals throughout the body and coordinating responses to internal and external stimuli. While there is a connection between the immune and nervous systems, white blood cells do not play a primary role in the functioning or regulation of the nervous system, which is primarily focused on neural communication and reflex actions.