Final answer:
IgM is the first antibody produced by B cells upon initial encounter with an antigen.
Step-by-step explanation:
IgM is the first antibody produced by B cells upon initial encounter with an antigen. It is initially produced in a monomeric membrane-bound form that serves as an antigen-binding receptor on B cells. The secreted form of IgM assembles into a pentamer with five monomers of IgM bound together by a protein structure called the J chain. IgM is important during the primary and secondary immune responses and can be used as a diagnostic marker for active or recent infections.