Final answer:
IgM is the correct answer as it is the antibody secreted into the blood during the early stages of a primary immune response, acting as the first line of defense before class switching leads to the production of other antibodies like IgG.
Step-by-step explanation:
The antibody that is secreted into the blood in the early stages of a primary response to an antigen is IgM. IgM is the first class of antibody made by plasma cells during primary immune responses. It is a pentamer, meaning it has five subunits, allowing it to bind to many identical antigens. Unlike IgG, which is a major antibody of late primary responses and the main antibody of secondary responses, IgM is produced first and is not as stable in its binding to antigens as IgG. The presence of IgM is crucial for early defense against a pathogen before the immune system has had enough time to undergo class switching and produce IgG, which is better at clearing pathogens from the blood and activating complement proteins. Therefore, the correct answer to the given question is option 5) IgM.