  In this paper we consider a very general U(1)-invariant field theory such
that a field operator commutes with its adjoint, what corresponds to a theory
of a charged bosonic particle. We show that from such an invariance follows the
existence of particles and antiparticles associated to the same physical state.
The field operator always turns out to be a sum of a particle creation and an
antiparticle annihilation operators. We study in detail the case when creation
and annihilation operators factorize and show that such operators are closely
related to q-deformed commutation relations.
