Final answer:
The follicular mantle zone is made up primarily of naïve IgD+ B cells that are waiting to encounter antigens, at which point they can become activated and participate in the immune response by producing antibodies.
Step-by-step explanation:
The follicular mantle zone primarily consists of naïve IgD+ B cells that are not participating in the response against the antigen. These B cells play a critical role in the immune system, as they are the precursors to the B cells that will eventually become activated and produce antibodies in response to pathogens. The follicular mantle zone, which is a component of secondary lymphoid organs such as lymph nodes, serves as an area for B cells to reside until they encounter their specific antigen.