Final answer:
IgM is the first and largest antibody produced during a primary immune response, and it excels at binding complement proteins. IgG, which is produced later, is the most abundant in serum, crosses the placenta for fetal protection, and is versatile in body defense. B cells can switch from producing IgM to IgG, retaining specificity to the pathogen.
Step-by-step explanation:
The first antibody produced in response to infection is IgM, which is also the largest antibody due to its pentameric form. During a primary immune response, IgM is the earliest antibody to appear and is adept at binding complement proteins, significantly contributing to an effective immune defense against pathogens.
As the immune response progresses, B cells can undergo class switching to produce other antibodies such as IgG, IgA, or IgE, which retain their specificity to the pathogen. IgG, in particular, is the most abundant antibody in human blood and is crucial for both primary and secondary immune responses.
Notably, IgG is the only antibody that can cross the placenta, providing passive immunity to the fetus, and also plays a versatile role in the body's defense mechanism due to its ability to activate complement and operate in tissue spaces.