BM from young rabbits is populated by osteoblasts which support B cell development. At this age, osteoblasts, pro-B cells, and pre-B cells are found at their highest numbers in rabbit BM. By 2 months of age these populations are greatly decreased. The BM microenvironment is the major contributor to the loss of B lymphopoiesis, which appears to be blocked at the rLP→pro-B cell stage. In >2 month old rabbits, the BM is filled with adipocytes. We hypothesize that adipocytes produce adipokines and DAMPs which lead to the accumulation of MDSCs which produce IL-1. We further hypothesize that IL-1 acts on rLPs, or yet to-be-defined MPPs, promoting myelopoiesis at the expense of B lymphopoiesis.