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

White blood cells, or leukocytes, play a critical role in the immune system by defending the body against infections and foreign invaders. They are an essential component of the circulatory system, as they travel through the bloodstream to reach sites of infection or injury. When pathogens such as bacteria and viruses invade the body, white blood cells respond by identifying, attacking, and neutralizing these harmful agents. This process is fundamental to maintaining the body's health and preventing illness.

In contrast, other functions mentioned serve different roles in the circulatory system. For example, transporting nutrients is primarily the job of red blood cells and plasma, while oxygen transportation is mainly conducted by hemoglobin within red blood cells. Maintaining blood pressure involves the regulation of blood volume and vessel constriction, which is also not the primary function of white blood cells. Therefore, the specific and well-defined role of white blood cells in fighting infections distinguishes them in the context of this question.