What is the role of the immune system in determining individually distinct body odours?

Genetically inbred mice and rats which are identical except for the genes of the major histocompatibility complex (MHC) produce unique urinary odours which can be discriminated by other animals. Congenic strains differing in both the Class I and Class II regions of the MHC produce distinct urinary odours. These urine odours can be used for mate selection and parental recognition, and it has been suggested that they provide a unique genetic mechanism for kin recognition. However, the non-MHC genes and the X and Y chromosomes also modulate the urinary odours of rodents, and rearing rats in a bacteria-free environment inhibits the production of unique MHC-related odours. We have found that dietary differences produce a greater effect on individual odours than differences at one MHC locus. These results suggest that the MHC, commensal bacteria, and dietary products interact to produce urinary odours which can be used for individual recognition in rodents. The problem is: what is the role of the immune system in determining individually distinct body odours? A model that suggests possible answers to this question is proposed.