CD38 and influenza: When analysing the phenotype of cross-reactive T cells, it became clear that they share characteristics of the two specific populations, displaying a high memory frequency similar to that of PeHA-322 reactive cells (consistent with expansion in response to previous influenza strains) together with strongly upregulated CD38 after vaccination (indicating activation by the 2014 vaccine) as seen for the TxHA-321-specific cells (Fig. 5c).