An abnormally decreased level of immunoglobulin in blood. The phenotype is HP_0004313 (Decreased circulating immunoglobulin concentration). Also known as: Decreased antibody level in blood, Decreased circulating antibody level, Decreased immunoglobulin level, Decreased serum immunoglobulin, Hypogammaglobulinemia, Immunoglobulin deficiency, Reduced immunoglobulin levels.