From this proof-of-concept study, we reason that (1) CJ:KREC ratio can be used to assess the severity of humoral PID and aid in differentiation between antibody deficiency syndromes and CVID, and (2) that the serum BAFF level can supplement the CJ:KREC ratio data to evaluate within humoral PID for lymphoproliferative features. This evidence concerns the gene TNFSF13B and inborn error of immunity.