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We report on highly sensitive label free optical biosensing of proteins based on nematic liquid crystals. The principles of biosensing are related to the change in the liquid crystal alignment induced by biomolecules adsorbed on the cell inner surface, which can be easily detected with a polarizing optical microscope. Although this approach is well known, we propose here an experimental strategy that allows us to reach a detection limit of the order of 10-13 g/ml, orders of magnitude higher than the one reported in literature for the same kind of optical biosensors. Furthermore, our method leads to assess a well-defined, specific dependence of protein concentration on cell birefringence, for rapid quantitative biosensing. The proposed biosensor can additionally be used for the detection of antibodies.