Unlike conventional memory T cells which are generated upon encounter with their cognate peptide antigens on classical MHC molecules, ‘innate’ memory T cells arise under homeostatic conditions in response to neonatal lymphopenia, high levels of IL-4 or exposure to self-antigens in naïve mice (Jameson et al., 2015; Sprent and Surh, 2011). This evidence concerns the gene IL4 and lymphopenia.