Final answer:
A pre-B cell becomes an immature B cell during the B cell maturation process in the bone marrow, which includes positive and negative selection to ensure functionality and self-tolerance. Once these steps are completed, the cells migrate to the spleen for final maturation into naïve mature B cells.
Step-by-step explanation:
A pre-B cell becomes an immature B cell during the process of B cell maturation in the bone marrow. The maturation process starts with stem cells differentiating into immune cells where a pre-B cell undergoes several stages including testing for functionality of its antigen-binding receptors through positive selection. Only those with normal functional receptors pass this initial test. Subsequently, the pre-B cell must escape negative selection designed to eliminate any self-reacting cells to prevent autoimmunity, either through apoptosis, receptor editing, or induction of anergy.
Upon passing these selection processes, the now immature B cells migrate to the spleen to undergo their final stages of maturation, becoming naïve mature B cells. These are still inactive but are fully capable of responding to antigens. The full activation occurs when they encounter an antigen, which they can then present on their MHC to helper T cells, receiving help in the form of cytokines to proliferate and differentiate into plasma cells that secrete antibodies or into memory B cells.