CD4 and infection: The amplitude of cross-reactive CD4+ T cells observed against H1, H2 and to a lesser extent H7 was consistent with the sequence homologies of H5 with those HA proteins (74.5, 62.5 and 41.7% for H2, H1 and H7, respectively).36 As aforementioned, influenza history (infection or vaccination) and its relatively short exposure history may also explain why cross-reactive Thpp T cells responses tend to be higher for H1 than for the other HA strains.