The differences between IgM and IgG are as follows: 1. IgM is the earliest specific antibody in the humoral immune response, with low levels in the body, and can be used for early diagnosis of infection, starting to be produced 3-5 days after infection, representing the body in the acute infection phase, but the antibody is short-lived and disappears quickly, lasting 2-3 weeks in the blood and not able to pass through the placenta. 2. It is the only immunoglobulin that can pass through the placenta. The content of IgG generally reaches its peak when IgM is close to disappearing and lasts longer in the blood, and may even exist for a long time, which usually means that the infection has existed in the body for some time, suggesting that it is in the middle and late stages of infection or has a previous infection.