  Photons with very high energy up to TeV (VHE) emitted from active galactic
nuclei (AGNs) provide some invaluable information of the origin of $\gam$-ray
emission. Although 66 blazars have been detected by {\it EGRET}, only three low
redshift X-ray selected BL Lacs (Mrk 421, Mrk 501, and 1ES 2344+514) are
conclusive TeV emitters (PKS 2155-304 is a potential TeV emitter) since VHE
photons may be absorbed by cosmological background infrared photons ({\it
external} absorption). Based on the ``mirror'' effect of clouds in broad line
region, we argue that there is an {\it intrinsic} mechanism for the deficiency
of TeV emission in blazars. Employing the observable quantities we derive the
pair production optical depth $\tgg(\eps_{\rm obs})$ due to the interaction of
VHE photons with the reflected synchrotron photons by electron Thomson
scattering in broad line region. This sets a more strong constraints on very
high energy emission, and provides a sensitive upper limit of Doppler factor of
the relativistic bulk motion. It has been suggested to distinguish the {\it
intrinsic} absorption from the {\it external} by the observation on variation
of multiwavelegenth continuum.
