Which organ is responsible for filtering blood and storing immune cells?

The spleen plays a crucial role in filtering blood and storing immune cells. It acts as a blood reservoir, helping to regulate the amount of blood in the circulatory system and removing old or damaged red blood cells. It also houses a significant number of lymphocytes, which are important for the immune response. These immune cells help to identify and fight infections and are essential in the body's defense mechanisms.

The liver, while important for detoxification and metabolism, does not primarily function in the filtering of blood in the way that the spleen does, nor is it a major storage site for immune cells. The kidneys are more focused on filtering blood to form urine and regulating fluid balance, but they do not store immune cells. The gallbladder is primarily involved in the storage of bile, which aids in digestion, and does not have a role in blood filtration or immune response. Therefore, the spleen is the organ that perfectly fits the description of filtering blood and storing immune cells.