BTK and B cell deficiency: X‐linked immunodeficient (XID) and BTK KO mouse: milder B cell deficiency than in XLA patients with 50% of the peripheral B cells remaining, loss of B1 B cells in the peritoneal cavity, immature phenotype of peripheral B cells, low levels of secretory IgM and IgG3, absent responses to T cell independent type II antigens 39, 40