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Surface biofunctionalization of capacitive micromachined ultrasonic transducer (CMUT) based sensor can develop non-informative stress and elasticity, which are distorting sensor readings related with the mass of established immune complex. The prototype sensor calibration function, using polimethylmetacrylate (PMMA) films from 7 to 100 nm, was developed to relate the sensor readings with the changing mass, elasticity and stress of the film on the CMUT surface. The range of calibration in terms of mass is from 1 to 80 pg per single CMUT cell, 0 - 20 MPa in terms of stress with corresponding sensor resonance frequency and resistance readings from 10 to - 35 kHz and from 4.5 to -0.5 Ω. Antigen and antibody specific interaction sensing experiment was performed and obtained results showed that informativity of CMUT based immunosensor is much greater than expected, since most of the sensor readings were out of the calibration function, developed for PMMA, range. At the same time, immunosensing experiment demonstrated clear distinction between antigen and antibody mass, which is more than threefold.