Most white blood cells are produced in the bone marrow and released into the bloodstream through which they circulate to all parts of the body, ready to fight any infection that they encounter. 

The spleen, a small organ just above the stomach on the inside of the left rib cage, acts as a reservoir, storing a supply of white blood cells, which can be released quickly into the bloodstream when needed to help fight infection or respond to any immune challenges. The lymph tissues are also involved with storing white blood cells.