The spleen is a fist-sized organ tucked behind the rib cage on the left side of the body. When it enlarges (medically known as splenomegaly), it can cause a dull, aching pain under the left rib cage.

The rib cage attaches to the breastbone and spine, and the ribs protect many vital organs. On the left side of the body, these organs include the: Costochondritis is inflammation of the cartilage ...