  Polarizability of non-interacting 2D Dirac electrons has a 1/\sqrt{qv-\omega}
singularity at the boundary of electron-hole excitations. The screening of this
singularity by long-range electron-electron interactions is usually treated
within the random phase approximation. The latter is exact only in the limit of
N -> infinity, where N is the ``color'' degeneracy. We find that the
ladder-type vertex corrections become crucial close to the threshold as the
ratio of the n-th order ladder term to the same order RPA contribution is
(\ln|qv-\omega|)^n/N^n$. We perform analytical summation of the infinite series
of ladder diagrams which describe excitonic effect. Beyond the threshold,
qv>\omega, the real part of the polarization operator is found to be positive
leading to the appearance of a strong and narrow plasmon resonance.
