Low levels of the immunoglobulin G (IgG) class antibody occurring when an infant is between 5 and 6 months of age. The maternal IgG that has crossed the placenta begins to drop after birth and reaches its lowest level (about 350 mg/dl) at this point. If IgG production is decreased, transient hypogammaglobulinemia develops. Normal blood levels of B cells, IgA, and IgM usually are present, which differentiates this transient disorder from hereditary, X-linked hypogammaglobulinemia. Some infants develop recurrent infections and must be treated with intravenous gamma globulin (IVIG) until IgG production increases.