  The effect of a mass-loading of the magnetohydrodynamic (MHD) flow in a
relativistic jet from active galactic nuclei (AGN) due to
$\gamma\gamma\rightarrow e^+e^-$ conversion is considered analytically. We
argue that the effects of charge average separation due to specific initial
pairs' motion lead to partial magnetic and electric fields screening or
enhancement. The effect of the fields screening has not been considered
earlier. The pairs with the center-of-mass moving faster or slower than the
bulk plasma flow create a surface charge and a current which either screen or
enhance both electric and magnetic fields in a pair creation domain. This
impacts the bulk flow motion, which either accelerates or decelerates. The
pairs with the center-of-mass moving with exactly the drift velocity does not
induce the fields disturbance. In this case the flow decelerates due to pure
mass-loading. For these different cases the Lorentz factor of the loaded
outflow is calculated as a function of loading pairs number density. The effect
may be important on sub-parsec to parsec scales due to conversion of TeV jet
radiation on soft infra-red to ultra-violet external isotropic photon field.
This leads to an outer shell acceleration. The conversion of MeV jet radiation
on larger scales may account for the flow deceleration due to pure
mass-loading. The proposed mechanism may be a source of internal shocks and
instabilities in the pair creation region.
