The immune system is expert at identifying infected cells and destroying them. It can do this because molecules called the Major Histocompatibilty Complex (MHC) bind short peptides from the infecting virus or bacterium and display them on the cell's surface. This enables the immune system to recognise an infected cell and destroy it.
Cells produce many different MHC molecules which are able to bind to different peptides and while some MHCs can bind many different peptides, others show greater selectivity. In a recent paper in eLife, Chappell et al have solved the structures of seven chicken MHCs bound to different peptides bound to different peptides to investigate the molecular nature of this different binding specificity. They also show that the MHCs which are more promiscuous are expressed in smaller amounts by both chicken and human cells.
The exact types of MHC molecules produced varies from individual to individual and this difference in peptide binding and expression level could explain why different individuals show different resistances to particular diseases.
