In X-linked agammaglobulinemia (XLA), BCR transcripts from peripheral blood CD19+ CD10+ IgM+ CD27−, emigrant mature naive B‐cells, represent the majority of peripheral blood B‐cells in XLA patients, and a higher proportion of which are self‐reactive and polyreactive antibodies and preferentially used VH1‐3 and VH4‐34 genes (132). Here, CD19 is linked to Bruton-type agammaglobulinemia.