Within the IEI classified as predominant antibody deficiency, we detected specific IgG+ RBD+ MBCs in half of them, with the exception of 1 patient with CVID, 1 patient with NFKB1 deficiency, 1 patient with NFKB2 deficiency, and 1 patient with suspected IEI without confirmed genetic diagnosis. The gene discussed is NFKB1; the disease is agammaglobulinemia.