A decreased level of serum immunoglobulins. It may be inherited or acquired. It is caused by decreased or inefficient production of immunoglobulins from B cells or by a decrease in the numbers of B cells themselves. Low levels of immunoglobulins will affect the immune system's ability to combat bacterial infection. Supplementation of immunoglobulins is needed to prevent worsening outcomes. The disease is agammaglobulinemia (MONDO_0015977, a Monarch Disease Ontology id). Also known as: IGHM, hypogammaglobulinemia, mu heavy chain deficiency.