Which organ system includes the spleen and lymph nodes?

The spleen and lymph nodes are key components of the lymphatic system, which plays a critical role in the body's immune response and fluid balance. The lymphatic system is comprised of various structures and organs that help protect and maintain the fluid environment of the body through the production and transportation of lymph, a fluid that contains infection-fighting white blood cells.

The spleen functions to filter blood, removing old or damaged red blood cells and helping to initiate immune responses when pathogens are detected. Lymph nodes serve as filtration points for lymph fluid and as sites for lymphocytes to gather and respond to infections.

Given this function and the identification of lymphatic structures, choosing the lymphatic system accurately reflects the primary systems in the body responsible for these organs and their functions. The circulatory system primarily deals with the transport of blood and nutrients, the nervous system is responsible for transmitting signals through nerve cells, and the respiratory system is involved in gas exchange; none of these options encompass the role and structures of the spleen and lymph nodes.