Final answer:
B cells switch isotypes by rearranging gene segments encoding the constant region of the antibody, while the variable region stays the same.
Step-by-step explanation:
When B cells are stimulated by cytokines secreted by T cells, they undergo class switching or isotype switching, which allows them to change from producing IgM to producing other antibody classes like IgG, IgA, or IgE. This process involves genetic rearrangement of gene segments encoding the constant region of the antibody, while the variable region, which determines the antibody's specificity, remains unchanged. As a result, B cells derived from the same activated B cell can produce different antibody classes with the same specificity.