Immunoglobulin G (IgG) deficiency is a condition that involves the immune system and is characterized by a shortage of type G antibodies. There are four different subclasses of IgG: IgG1, IgG2, IgG3, and IgG4. People with IgG deficiency may have a decreased amount of one or more of the IgG subclasses or a shortage of other types of antibodies, which may occur in various conditions including X-linked agammaglobulinemia, common variable immunodeficiency, and hyper-IgM syndrome. Individuals with IgG deficiency may have an increased susceptibility to infection, but the symptoms of this condition are variable and some people never experience any symptoms. Treatment may involve antibiotics and intravenous (IV) immunoglobulin.