Hey Bobby,
It's called an extramedullary haematopoiesis. These can grow as masses, usually found in the abdomen and can sometimes become a problem if they expand and put pressure on the spine or an organ. Often they cause no problem. This can also take place at the cellular level and will show the same result as lymphoma in cancer scans, so thal patients need to advise their doctors of this if a scan ever shows lymphoma.