Which of the following is true about the immune system cells?

The immune system is a sophisticated network of cells and proteins that play a crucial role in defending the body against pathogens. One of the key features of immune system cells is their mobility. They do not remain static; instead, they actively travel throughout the body to identify and respond to threats. These cells can move through both the circulatory system and the lymphatic system, which allows them to reach various tissues and organs where they are needed. This ability to traverse these fluids is essential for effective immune surveillance and response, ensuring that immune cells can quickly reach sites of infection or inflammation.

The other options do not accurately represent the characteristics of immune system cells. For example, while some immune cells do communicate through chemical messengers (cytokines), the statement suggesting communication solely through hormonal signals is limited. Additionally, although many immune cells originate in the bone marrow, some are produced in other areas, such as the thymus for T-cells. Therefore, the mobility of immune cells through circulatory and lymphatic fluids is a defining feature that enables them to function effectively within the immune system.