  In this paper we examine the 4-dimensional effective theory for the light
Kaluza-Klein (KK) modes. Our main interest is in the interaction terms. We
point out that the contribution of the heavy KK modes is generally needed in
order to reproduce the correct predictions for the observable quantities
involving the light modes. As an example we study in some detail a
6-dimensional Einstein-Maxwell theory coupled to a charged scalar and fermions.
In this case the contribution of the heavy KK modes are geometrically
interpreted as the deformation of the internal space.
