A spleen was an organ involved in the production and removal of blood cells in most vertebrates and forming part of the immune system.
The Fourteenth Doctor scanned his own spleen which confirmed all his vitals were normal. (COMIC: Liberation of the Daleks)
In the 1890s, a Clockwork Droid scanned the Twelfth Doctor, and listed organs they could harvest, as items on the menu at Mancini's Family Restaurant: liver, spleen, brain stem, eyes, lungs and skin. (TV: Deep Breath)
Alfred took the spleen, kidneys and liver of his first victim with the intention of using them to revive Julius Quilter when he died. (HOMEVID: Unnatural Selection)