  Using electromagnetic interaction as an example, response transformations
[L.P. and S.S., Ann.Phys. 323, 1963, 1989 (2008), 324, 600 (2009)] are applied
to the standard perturbative approach of quantum field theory. This approach is
rewritten in the form where the place of field propagators is taken by the
retarded Green function of the field. Unlike in conventional
quantum-field-theoretical techniques, the concept of space-time propagation of
quantized field is built into our techniques.
