What diagnosis is suggested by flow cytometry showing a large population of T lymphocytes and a relative absence of IgM in a child with recurrent infections?

• A. Adenosine deaminase deficiency
• B. Hyper-IgM syndrome
• C. IgA deficiency
• D. X-linked agammaglobulinemia

Answer: (D)

The scenario describes a child with recurrent infections, characterized by a flow cytometry result indicating a large population of T lymphocytes and a relative absence of IgM. This is indicative of X-linked agammaglobulinemia, a condition caused by a mutation in the BTK gene that leads to the failure of B cell development.

In X-linked agammaglobulinemia, the immature B cells in the bone marrow do not mature into functional B cells, resulting in a profound deficiency of immunoglobulins, including IgM, IgG, and IgA. The presence of T lymphocytes is normal; however, the critical defect lies in the absent or severely reduced number of circulating B cells and their resultant antibody production capability.

Recurrent infections typically arise due to the inability to produce adequate antibodies to respond to pathogens, particularly encapsulated bacteria. The lack of IgM, which is the first antibody produced in response to an infection, further supports this diagnosis.

Other conditions listed, such as adenosine deaminase deficiency, hyper-IgM syndrome, and IgA deficiency, have distinguishing features that do not align with the finding of a large population of T lymphocytes coupled with a marked deficiency of IgM. For example