What is the role of white blood cells in the immune system?

White blood cells, also known as leukocytes, play a crucial role in the immune system by defending the body against pathogens such as bacteria, viruses, fungi, and parasites. They are a key component of the body's defense mechanisms, actively seeking out and destroying harmful invaders. There are several types of white blood cells, each with specialized functions, including phagocytes, which engulf and digest pathogens, and lymphocytes, which can target specific pathogens and help in establishing immunity.

In contrast, the other roles described in the options are associated with different components of the blood or bodily functions. Promoting blood clotting is primarily the function of platelets and certain plasma proteins. Transporting carbon dioxide is mainly carried out by red blood cells, which carry oxygen and carbon dioxide in the bloodstream. Maintaining blood volume is a function influenced by plasma proteins and the fluid balance in the body, rather than a direct role of white blood cells.