Final answer:
Pro-B cells become pre-B cells when they undergo differentiation and maturation in the bone marrow. This is triggered by the interaction between a B cell and its matching antigen, leading to the release of cytokines by helper T cells to signal the development of the B cell into a plasma cell.
Step-by-step explanation:
Pro-B cells become pre-B cells when they undergo differentiation and maturation in the bone marrow. During this process, up to 100 trillion different clones of B cells are generated, leading to a diverse repertoire of antigen receptors. Once a B cell encounters its matching antigen, it engulfs and digests it, presenting fragments of the antigen on its surface. This attracts a helper T cell, which binds to the B cell and releases cytokines that signal the B cell to develop into a plasma cell.