Fig. 3 ∣. Convenience of tissue redistribution.

The ability of memory cells to reside in tissues facilitates an accelerated immune response at the site of pathogen entry without having to migrate. Top, small-intestine ILC2s (siILC2s) have been shown to differentiate and migrate into the lung in response to Nippostrongylus brasiliensis in a sphingosine-1-phosphate-dependent manner (blue gradient). A small fraction of these iILC2s then stays in the lung (Tr-iILC2s) and co-exists with lung ILC2s, whereas other iILC2s migrate back into the small intestine. Bottom, in addition to tissue-resident helper ILCs, splenic-tissue-resident NK cells that contribute to the MCMV-specific response have been described (black arrow). Furthermore, although other tissue-resident NK cells can be generated in response to MCMV and other infections, it is unclear whether they are primed elsewhere, such as SLOs (blue arrow), or in the tissues in which they ultimately reside (red arrow).