ICOS and common variable immunodeficiency: They are characterized by a Tfh cell-liketranscriptome, with excessive numbers of Tfh cells and germinal centers (51,57).Humans who are deficient in ICOS develop common variable immunodeficiency, in whichthere is impairment in the development of memory B cells and immunoglobulin class switchdoes not occur (58), highlighting the importanceof the ICOS-ICOSL interaction for the development of an effective humoral response.