Final answer:
IgM is the first circulating immunoglobulin to respond to an antigen and the first antibody produced by a newborn. It appears as a monomer on naive B cells and becomes a pentamer in plasma. In contrast, IgG is prevalent in late primary and secondary responses and can cross the placenta to provide fetal immunity.
Step-by-step explanation:
The immunoglobulin that is the first circulating antibody to appear in response to an antigen is IgM. IgM, whose monomer acts as a surface receptor for naive B cells, is initially produced in its monomeric form. When secreted into blood plasma during a primary immune response, it assembles into a pentamer. This pentameric form of IgM is crucial because it possesses multiple antigen-binding sites, making it effectively the first line of defense against pathogens. As a newborn's immune system begins to respond to pathogens, IgM is also the first type of antibody produced. In contrast, IgG is a major antibody of late primary responses and often dominates during secondary immune responses. IgG's versatility allows it to clear pathogens from the blood and activate complement proteins. Moreover, IgG is unique in that it can cross the placenta, providing passive immunity to the developing fetus.