  We study the renormalizable singlet-doublet fermionic extension of the
Standard Model. In this model, the new vector-like fermions couple to the gauge
bosons and to the Higgs via new Yukawa couplings, that allow for nontrivial
mixing in the new sector, providing a stable, neutral dark matter candidate.
Approximate analytic formulae are given for the mass spectrum around the blind
spots, where the dark matter candidate coupling to $h$ or $Z$ vanishes. We
calculate the two particle scattering amplitudes in the model, impose the
perturbative unitarity constraints and establish bounds on the Yukawa
couplings.
