Immunology - Unmasking the killer's accomplice

One way in which our immune system detects virally infected cells is by monitoring levels of class I major histocompatibility complex (MHC). In infected cells, class I expression is downregulated, acting as a signal to the natural killer (NK) cells to target that cell for destruction. But some class I homologues stimulate — rather than inhibit — the NK cells, and one group has now found out why. They have discovered a signal-transducing adaptor molecule called DAP12, which couples recognition of class I MHC to an activating signal-transduction pathway.