The Imd pathway is a broadly-conserved NF-κB immune signalling pathway of insects and some arthropods that regulates a potent antibacterial defence response. The pathway is named after the discovery of a mutation causing severe immune deficiency (the gene was named "Imd" for "immune deficiency"). The Imd pathway was first discovered in 1995 using Drosophila fruit flies by Bruno Lemaitre and colleagues, who also later discovered that the Drosophila Toll gene regulated defence against Gram-positive bacteria and fungi. Together the Toll and Imd pathways have formed a paradigm of insect immune signalling; as of September 2, 2019, these two landmark discovery papers have been cited collectively over 5000 times since publication on Google Scholar.
The Imd pathway responds to signals produced by Gram-negative bacteria. Peptidoglycan recognition proteins (PGRPs) sense DAP-type peptidoglycan, which activates the Imd signalling cascade. This culminates in the translocation of the NF-κB transcription factor Relish, leading to production of antimicrobial peptides and other effectors. Insects lacking Imd signalling either naturally or by genetic manipulation are extremely susceptible to infection by a wide variety of pathogens and especially bacteria.