Sep 29, 2009 · 1. IgM is the immediate antibody that is produced once a human body is exposed to a bacteria, virus or a toxin 2. IgG is found throughout the body, mainly in most of the bodily fluids, while IgM is found mainly in the blood and lymphatic fluids. 3. IgM is larger in size compared to IgG 4. IgM is temporary and disappears after a few weeks.