Final answer:
IgM is the immunoglobulin, like IgG, that can fix complement and is the main antibody of primary immune responses, with a pentameric form that binds multiple antigens.
Step-by-step explanation:
The immunoglobulin that, like IgG, can fix complement is IgM. IgM is the main antibody produced in the primary immune response and is very effective at fixing complement due to its pentameric form, which allows it to bind to multiple antigens simultaneously. IgM is crucial for initial defense against pathogens and is the first antibody made in the blood plasma during primary responses. Although IgG is a major antibody of late primary responses and the main antibody of secondary responses that also can activate complement proteins, it is not as effective as IgM in complement fixation.