Why is the immune system not considered one of the body's organ systems?

The immune system is not classified as one of the body's organ systems because it is not comprised of a single, cohesive group of organs working together in a specific anatomical location. Instead, it consists of a network of cells, tissues, and organs, including lymph nodes, spleen, and bone marrow, that are dispersed throughout the body and work collaboratively to defend against pathogens. This decentralized nature makes it more of a functional system than a distinct organ system like the cardiovascular or respiratory systems.