A virus, after infecting a host cell, is inside a protective layer of the cell, hidden from various immune system receptors. The host cell instead starts showing their “symptoms” to other cells using Major Histocompatibility complex proteins. But, the viruses, have evolved ways to spread and replicate themselves, evading detection by T-cells as some viral mechanisms inhibit the MHC expression, which then cannot be detected by T-cells. To combat such “irregular” cells, there are Natural Killer cells, NK Cells which detect, isolate and then kill such cells that display less than required MHC proteins on their cell surface. Similarly, host cells respond via interferons, antibodies, and antibody-mediated phagocytosis.