The Spleen Stores Monocytes

Shortly after severe tissue damage such as that caused by a heart attack or an infection, the number of monocytes, the white blood cells that eventually mature into macrophages, increases dramatically in the blood. These new monocytes appear too quickly to have been newly produced from stem cells in bone marrow. So where do they come from?

Apparently they come from the spleen. The spleen stores up to ten times as many monocytes as there are in the bloodstream at any one time. When a tissue is injured the spleen releases its stored monocytes, which then migrate to the site of injury, develop into macrophages, and participate in the cleanup and repair process. It’s a pretty efficient use of resources, when you think about it - a virtual army of monocytes is kept on standby, ready to be deployed when needed.