An occupant detection system comprises first and second electric field sensors mounted in or on a seat back of a seat, and at least one seat weight sensor. The peptidic linker group is a peptide group, preferably with from 2 to 15 amino acids, which has binding specificity for keratin-containing material. In an alternative embodiment, the coupling layer comprises a material like lead-glass that is transparent for light and absorbing for X-radiation.