Why is my spleen growing back after splenectomy?

Splenosis: Small fragments of splenic tissue or splenules can grow after the spleen is removed.

Accessory spleen: About 10 percent of people have an accessory spleen. In some cases, this can also be due to part of the original spleen taking hold and growing as an autotransplant, also called splenosis. These accessory spleens can be functional, and depending on the reason for the original splenectomy, this may be of clinical significance or not.