Final answer:
Immature B cells coexpress IgM and IgD as they pass self-tolerance tests, which involve both positive and negative selection processes, including central and peripheral tolerance mechanisms, leading to maturation into plasma cells.
Step-by-step explanation:
Immature B cells that pass the self-tolerance test are identified by the coexpression of IgM and IgD. This occurs through a series of selective processes including both positive and negative selections. Positive selection ensures that B cells with functional receptors proceed in the maturation process. Negative selection leads to the elimination or inactivation of self-reacting B cells to prevent autoimmune responses.
During the maturation, B cells undergo central tolerance mechanisms in the bone marrow, where they are tested against self-antigens. B cells that react strongly with these self-antigens are prompted to commit apoptosis (clonal deletion) or become non-functional (clonal anergy). Conversely, peripheral tolerance takes place for B cells that leave the bone marrow functional but encounter self-antigen without receiving necessary signals from helper T cells (Th2), leading to their apoptosis.
For B cells to differentiate into mature antibody-secreting plasma cells, they require signals post antigen recognition, which involves T cell-dependent or independent pathways. This maturation is facilitated by complex genetic rearrangements within B cells, giving rise to a diverse repertoire of antigen-specific receptors.