A rare case of TCF3-related agammaglobulinemia in a child with recurrent Haemophilus influenzae bacteremia.

We report a case of a previously healthy 4-year-old girl who presented with two recurrent episodes of complicated pneumonia and Haemophilus influenzae bacteremia. Immunologic evaluation revealed hypogammaglobulinemia, as immunoglobulins (IgG, IgA, IgM, IgE) were below detectable levels and genetic testing identified a pathogenic TCF3 variant, confirming a diagnosis of TCF3 (transcription factor 3) deficiency. TCF3 is a key transcription factor in B- and T-cell development, and its deficiency results in impaired antibody production and increased susceptibility to infections. This case highlights the rarity of TCF3 mutations in pediatric patients, the importance of early recognition, and the critical role of immunoglobulin replacement therapy as the current mainstay of treatment.