 There are stages such as Pre-B-I cells (5% to 10% of the total); Pre-B-II cells (60% to 70%) while the remaining 20% to 25% are immature B cells.Most textbooks say that B Cells mature in the bone marrow but, generally, immature B cells migrate to the spleen for 'higher education' of some sort where they go through transitional stages before final maturation.