Low energy theorem for scalar and vector interactions of a composite   spin-1/2 system
  Scalar and vector interactions, with the scalar interaction coupled to a composite spin-1/2 system so as to cause a shift of its mass, are shown to obey a low-energy theorem which guarantees that the second order interaction due to z-graphs is the same as for a point Dirac particle. Off-shell and contact interactions appropriate to the composite system cancel and this is verified in a model of a composite fermion. The result provides a justification for the use of the Dirac equation as it has been in relativistic nuclear scattering and mean field theories. 
S. J. Wallace, Franz Gross and J. A. Tjon