The spleen and thymus are lymphoid organs with very different jobs. The spleen filters the blood, while the thymus trains T cells, mostly during childhood.
Where the spleen is
The spleen lies in the left upper abdomen under the 9th to 11th ribs, behind the stomach. A normal spleen is about the size of a fist and usually cannot be felt.
What the spleen does
Its red pulp removes old or damaged red blood cells and holds a reserve of platelets. Its white pulp holds lymphocytes that react to germs in the blood, especially encapsulated bacteria such as pneumococcus.
Life without a spleen
After the spleen is removed, people have a lifelong risk of overwhelming infection with encapsulated bacteria. They need specific vaccines and urgent assessment for any fever.
The thymus
The thymus sits in the upper chest behind the sternum, in front of the heart and great vessels. It is large in children, where T cells learn to react to foreign antigens and ignore the body's own, and it shrinks and fills with fat after puberty.
At the bedside
The spleen is soft and full of blood, so left upper abdominal pain or left shoulder tip pain after trauma can signal splenic rupture and internal bleeding. An enlarged spleen is felt below the left costal margin as the patient breathes in.
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Written by Making Synapses. Topic reference: OpenStax, Anatomy and Physiology 2e (no text adapted).