The spleen is a lymphatic system organ found in all vertebrate animals. In humans, the spleen is located in the upper left side of the abdomen, above the stomach and under the ribs. It has both hematopoietic and immunologic functions, in that it acts as a blood filter, produces blood cells, and destroys old and damaged cells. In adult, the spleen is responsible for B-cell maturation.
The embryonic origins of the spleen are obscure, with most studies describing the earliest rudiment of the spleen as a condensation of mesodermal mesenchyme which takes place at E10.5, adjacent to the splanchnic mesodermal plate.