A classification of dysgammaglobulinemias characterized by low or undetectable serum levels of one of the four immunoglobulin class G (IgG) subclasses. Selective IgG1 deficiency is rare and primarily decreases the immune response to bacterial protein antigens. Selective IgG2 deficiency is the most common subclass deficiency among children and primarily leads to an inadequate response to bacterial polysaccharide antigens. Selective IgG3 deficiency is the most common subclass deficiency among adults and also primarily lowers the response to bacterial proteins. Selective IgG4 deficiency may be a clinically insignificant developmental variant, as IgG4 is a subclass that is virtually undetectable until the end of the first decade of life. Low levels of any IgG subclass will reduce the immune system's effectiveness and thus the clinical presentation of these diseases is usually recurrent infection, particularly by encapsulated bacteria.