Subsequently, we sought to investigate whether there is a difference in the serum levels of CXCL13 in patients with CVID: (a) between those with normal frequencies of CD21low B cells (CD21norm) and those with high frequencies of CD21low B cells (CD21low), and (b) between those with normal frequencies of switched memory B cells (smB+) and those with low frequencies (smB-). This evidence concerns the gene CXCL13 and common variable immunodeficiency.