  We generalize the concept of conserving,\Phi-derivable, approximations to
relativistic field theories. Treating the interaction field as a dynamical
degree of freedom, we derive the thermodynamical potential in terms of fully
dressed propagators, an approach which allows us to resolve the entropy of a
relativistic plasma into contributions from its interacting elementary
excitations. We illustrate the derivation for a hot QED plasma of massless
particles. We also discuss how the self-consistency of the treatment manifests
itself into relationships between the contributions from interaction and matter
fields.
