Methods for monitoring immune status of a subject

A method of assaying for soluble B-cell maturation antigen (sBCMA) polypeptide for monitoring immune status of a subject, comprising:

• (a) providing a first biological sample obtained from a subject diagnosed with an immunodeficiency disease selected from the group consisting of a X-linked agammaglobulinemia (XLA), Common Variable Immune Deficiency (CVID), IgG Deficiency, IgA Deficiency, IgM Deficiency, or Hyper IgM Syndrome;
• (b) providing a second biological sample obtained from the subject after the first biological sample was obtained;
• (c) detecting a first amount of sBCMA polypeptide or fragments thereof by binding the sBCMA polypeptide or fragments thereof in the first biological sample;
• and
• (d) detecting a second amount of sBCMA polypeptide or fragments thereof by binding the sBCMA polypeptide or fragments thereof in the second biological sample,
• wherein the first biological sample and the second biological sample are each independently a serum or plasma sample or supernatant obtained from a culture of the subject's bone marrow mononuclear cells or peripheral blood mononuclear cells,
• wherein the first biological sample is obtained from the subject prior to administration of a treatment regimen for X-linked agammaglobulinemia (XLA), Common Variable Immune Deficiency (CVID), IgG Deficiency, IgA Deficiency, IgM Deficiency, or Hyper IgM Syndrome, and the second biological sample being obtained after administration of the treatment regimen for X-linked agammaglobulinemia (XLA), Common Variable Immune Deficiency (CVID), IgG Deficiency, IgA Deficiency, IgM Deficiency, or Hyper IgM Syndrome to the subject.