X-linked agammaglobulinemia (XLA)

Background

X-linked agammaglobulinemia (XLA) is the most common cause of agammaglobulinemia present from birth. Patients with XLA lack antibodies, they are prone to developing infections, which frequently occur at or near the surfaces of mucus membranes, such as the middle ear, sinuses and lungs, but in some instances can also involve the bloodstream or internal organs.

Clinical

It is commonly caused by a mutation or deletion in the BTK gene which prevents the normal development of B lymphocytes, resulting in a severe antibody deficiency. The mutated gene is located on the X chromosome, so only boys are affected.

Testing

XLA testing is usually recommended for any child with recurrent or severe bacterial infections, particularly if the patient has small or absent tonsils and lymph nodes. First, an evaluation of serum immunoglobins may be conducted. In most patients with XLA, all the immunoglobulins (IgG, IgM and IgA) are markedly reduced or absent with some exceptions. If the serum immunoglobulins are low or if the physician strongly suspects the diagnosis of XLA, the B-cell number in the peripheral blood may be measured using flow cytometry. A low percentage of B-cells (nearly absent) in the blood is the most characteristic and reliable laboratory finding in XLA or Autosomal Recessive Agammaglobulinemia (ARA) patients. The diagnosis of XLA can be confirmed by examining the absence of BTK protein in monocytes or platelets or by detecting a mutation in the BTK gene

Treatment

The early diagnosis of XLA patients is highly desirable because the incidence of chronic lung disease in these patients results from delayed diagnosis and is one of the most detrimental factors on the prognosis and quality of life. Most patients who receive immunoglobulin or prophylactic antibiotics on a regular basis can have relatively normal lives.

Inheritance

The incidence of XLA is approximately 1 in 250,000 male live births and has no ethnic predisposition.

References

• Smith and Berglöf, X-Linked Agammaglobulinemia, GeneReviews, accessed on Jul 2018
• Winkelstein et al., X-Linked Agammaglobulimnemia, Report on a United States Registry of 201 Patients, Medicine, 2006
• Shillitoe and Gennery, X-Linked Agammaglobulinaemia: Outcomes in the modern era, Clinical Immunology 2017
• Borte et al., Neonatal screening for severe primary immunodeficiency diseases using high-throughput triplex real-time PCR, Blood Journal, 2011
• Barbaro et al., Newborn Screening for Severe Primary Immunodeficiency Diseases in Sweden—a 2-Year Pilot TREC and KREC Screening Study, J Clin Immunol, 2017