In this article. The spleen is a delicate, fist-sized organ under your left rib cage near your stomach. It contains special white blood cells that destroy bacteria and help your body fight infections. The spleen also makes red blood cells and helps remove, or filter, old ones from the body's circulation. A layer of tissue entirely covers the spleen in a capsule-like fashion, except where veins and arteries enter the organ. This tissue, called the splenic capsule, helps protect the spleen from direct injury.