IgG antibodies are one of the immunoglobulin isoforms, which has four subclasses, and the only isoform that can cross the placenta to provide passive immunity to the fetus.IgG antibodies play a role in the immune response by activating complement and neutralizing a variety of toxins. IgG antibodies are the only antibodies that can cross the placenta to protect the fetus during a mother’s pregnancy. Human IgG is divided into four main subclasses, which, depending on their concentration in the serum, are IgG1, IgG2, IgG3, and IgG4. IgG is synthesized by plasma cells, such as those in the spleen and lymph nodes, beginning in the first three months of life, and can approach the level of the adult at three to five years of age. The igG antibody is the main antibody produced by the humoral immune response and has an important immune effect. They can activate the classical pathway of complement, mediate bacteriolysis and cytotoxicity; mediate ADCC effects; modulate phagocytosis; bind SPA; and neutralize toxins and viruses. Plays an important role in anti-infective immunity, especially in the re-immune response.