ICOS and common variable immunodeficiency: Mutations in the ICOS gene cause ICOS deficiency, presenting with common variable immunodeficiency (CVID) including splenomegaly, autoimmune manifestations, recurrent bacterial infections, and IBD.89 Absence of ICOS results in abrogation of germinal center formation leading to severe reduction of class‐switched memory B cells, as well as reduction in naïve B cells.