Final answer:
IgM is the first immunoglobulin isotype produced during a primary immune response, characterized by pentameric structure with multiple antigen-binding sites.
Step-by-step explanation:
The first immunoglobulin isotype produced during the course of a primary immune response is IgM. This antibody is produced and secreted by B cells as the initial defense against pathogens. IgM starts as a monomeric membrane-bound receptor on naive B cells.
Once secreted, it forms a pentamer, which refers to the arrangement of five monomers joined together by a J chain. This pentameric IgM can bind to ten identical antigens, providing a strong early immune response before class switching occurs, which leads to the production of other antibodies such as IgG.
Although it does not bind antigens as stably as IgG, IgM's multiple antigen-binding sites make it an essential component of the body's initial adaptive immune response.