Heterozygous TCF3-Related Disease Presenting as X-linked Agammaglobulinemia Mimicry in a Male Toddler with B-cell Aplasia, Agammaglobulinemia, and Severe Neutropenia

Disease associated with TCF3 variants are rare and associated with autosomal dominant inheritance agammaglobulinemia. Patients present with profound reduction in CD19+ cells and a block in B cell development at the common lymphoid precursor to pro–B cell phase of differentiation. Severe neutropenia is frequently described in X-linked agammaglobulinemia (XLA) patients but not yet described as a presenting finding in TCF3-related disease. This case describes a 13-month-old full term male with a history of reactive airway disease and recurrent infections including multiple bouts of acute otitis media, hand -foot-mouth, and viral upper respiratory illnesses who presented with fever and persistent severe neutropenia (absolute neutrophil count