X-linked agammaglobulinemia is a primary immunodeficiency characterized by which feature?

• A. Absent or very low B cells and severe hypogammaglobulinemia
• B. Elevated IgM with normal B cells
• C. Absence of T cells with preserved B cells
• D. Overproduction of immunoglobulins causing autoimmunity

Answer: (A)

X-linked agammaglobulinemia results from a mutation in Bruton's tyrosine kinase (BTK) that blocks B cell maturation. Because B cells cannot progress from precursor stages to mature B cells, there are essentially no circulating B lymphocytes and antibody-producing plasma cells. This leads to very low or absent levels of all immunoglobulin isotypes (severe hypogammaglobulinemia). The humoral immune system is thus severely compromised, explaining the recurrent bacterial infections seen after maternal antibodies wane around 6 months of age. T cell numbers and function are typically normal, helping distinguish this condition from combined immunodeficiencies. The hallmark feature—absent/very low B cells with profound hypogammaglobulinemia—best describes X-linked agammaglobulinemia.