IgG and IgM are both antibodies, which are important effector molecules mediating humoral immunity. They are immunoglobulins produced by plasma cells that are proliferated and differentiated into B cells by the immune system in response to antigen stimulation and can bind specifically to the corresponding antigen. Among them, Ig stands for immunoglobulin, IgG and IgM are two different types of antibodies with the following differences: 1. Different in vivo content: IgG is the highest content of Ig in serum and extracellular fluid, accounting for about 75%-80% of total serum Ig; IgM accounts for 5%-10% of total serum Ig; 2. Different generation time: IgG starts to be synthesized 3 months after birth and approaches 3-5 years old IgM is more likely to activate complement than IgG and is the earliest antibody synthesized and secreted during individual development, and IgM can be produced in fetuses at late embryonic development; 3. Different clinical significance: IgG is the only immunoglobulin that can pass through the placenta, can be transferred to trophoblast cells, and actively enters the fetal IgM is also the earliest antibody to appear in the initial humoral immune response, and the detection of pathogen-specific IgM in serum suggests a recent infection and can be used for early diagnosis of infection. And in case of elevated IgM levels in cord blood, it indicates intrauterine infection in the fetus, such as rubella virus or cytomegalovirus.