X-linked agammaglobulinemia (XLA) is usually considered a disorder of B cell development;
however, the gene that is defective in XLA encodes a cytoplasmic tyrosine kinase called Btk,
that is expressed throughout myeloid as well as B cell differentiation. A review of medical
records of patients in whom mutations in Btk had been identified indicated that 13 of 50
patients (26%) had experienced episodes of profound neutropenia. In 12 of the 13 patients,
neutropenia was part of the acute illness that precipitated an evaluation for …