Final answer:
Most antibodies early in the primary immune response are of the IgM class, which is effective in the initial defense against pathogens. Later, B cells may switch to produce IgG and other antibodies for a more tailored response.
Step-by-step explanation:
Early in the primary immune response, most antibodies are of the IgM class. IgM antibodies are the first to be produced when an antigen is encountered by the immune system for the first time. These antibodies are assembled into pentamers, allowing them to bind up to ten identical antigens, which provides an effective initial defense against pathogens while the body prepares a more tailored response. As the immune response progresses, B cells can undergo class switching to produce other antibody classes like IgG, IgA, or IgE, with IgG being the main antibody in the late primary and secondary responses, due to its ability to clear pathogens from the blood more effectively and its capacity to cross the placenta to protect the developing fetus.