According to WebMD, the IgG and IgM antibodies differ in size, location and specific function. Antibodies are produced by the body's immune system in response to invaders such as viruses, bacteria, fungus, animal dander and cancer.
WebMD indicates that all bodily fluids contain IgG antibodies. They are the smallest and most common of the body's five types of antibodies. WebMD specifies that they are essential in the fight against viruses and bacteria, so up to 80 percent of antibodies are IgG. WebMD also explains that IgG antibodies are the only antibodies that are able to move from a mother to her fetus. KidsHealth states that IgM antibodies are located primarily in the blood and lymph fluid. They are the first line of defense when a new infection appears. IgM antibodies are the largest type of antibodies, according to WebMD.