The spleen is the organ that is responsible for both the storage and purification of red blood cells. It is positioned in the left upper abdomen, and is the largest organ of the lymphatic system. The spleen serves a critical role in immune function because it purifies the blood and helps the immune system to recognize and attack foreign pathogens and allergens. The spleen is composed of the red pulp and white pulp. The white pulp produces and grows immune cells as well as blood cells. On the other hand, the red pulp is responsible for purifying the blood and removing dead or old blood cells.