The body system that functions to maintain fluid balance, support immunity, and contains the spleen is the:

The Lymphatic System is responsible for maintaining fluid balance and supporting immunity. It contains organs like the spleen, tonsils, thymus, lymph nodes, and lymph vessels. The spleen, a part of the lymphatic system, plays a crucial role in filtering blood and storing blood cells. The Digestive System is primarily involved in the breakdown and absorption of nutrients, not fluid balance or immunity. The Urinary System is responsible for filtering waste products from the blood and regulating fluid balance, but it does not support immunity or contain the spleen. The Respiratory System is focused on gas exchange and oxygenating the blood, not fluid balance or immunity.