Final answer:
Cell surface IgD and IgM are antigen receptors on circulating B cells that haven't encountered antigen. They serve as B-cell receptors (BCRs) and play a key role in recognizing antigens and initiating an immune response.
Step-by-step explanation:
On circulating B cells that haven't encountered antigen, the antigen receptors are referred to as cell surface IgD and IgM.
These receptors, also known as B-cell receptors (BCRs), are membrane-bound monomeric forms of IgD and IgM that are expressed on the surface of naive B cells. They play a crucial role in recognizing antigens and initiating an immune response.