The spleen was an organ of the body that acted primarily as a blood filter.
Strain on the spleen was one of the side-effects of an experimental chemical stimulant designed to speed up reproduction and replacement of blood used in the mid-2260s. (TOS: "Journey to Babel")
In 2269, Leonard McCoy suffered hemorrhage of the spleen after being tortured by the Vians. (TOS: "The Empath")