The IgG subclasses are known to have different structure and functions. The IgG1 and IgG3 bind to monocyte and neutrophils and activate the complement more easily then IgG2 and IgG4. The levels of IgG subclasses found in newborns are mainly determined by the transplacental passage since the synthesis, in the first time of life, is very low. The levels found in adults are reached only during the adolescence. The immune response to a protein antigen is mainly in the IgG1 subclass, on the contrary the response to polysaccharide antigens is mainly IgG2. For that reason children, who produce very few IgG2 till they are 2 years old, cannot be vaccinated with carbohydrate vaccines unless a protein conjugate vaccine is used. In addition, the route of subministration, the dose of antigen, the age of vaccinated people and genetic factors can modify the subclass pattern obtained in response to vaccinations. For these reasons, probably, the immune response studied after a vaccination or in individuals who recovered from a natural disease is not always the same. Since the role of the different subclasses is not yet completely clarified, the interpretation of the importance of a selective "choice" of a subclass instead of another after vaccination is still difficult.