MICA and infection: Thus, NKG2D recognizes the stress-induced ligand MHC class I polypeptide-related sequence A (MICA), while NKp46 and NKp44 can directly bind influenza hemagglutinin, a response that is key to the role of NK cells in protecting against infection by the virus (Arnon et al., 2001; Mandelboim et al., 2001).