Temporal information of antigen concentration is valuable especially for those samples whose concentrations fluctuate. Concentration of airborne house dust mite (HDM) allergens changes over time while being influenced by environmental conditions or human activity, and it is important to monitor this change because exposure to such allergens should be avoided for prevention of allergic diseases. In general, it is challenging for immunosensors to be used for monitoring because immunocomplex formed on the sensor surface must be once broken to regenerate the surface for a next measurement, which often causes denaturation of antibodies and thereby dysfunction of the sensor. In this chapter, we describe a reusable immunosensor for monitoring of a HDM allergen—Dermatophagoides farinae group 1 (Der f 1). The immunosensor was fabricated using a surface acoustic wave (SAW) device and pH-tolerant protein scaffold (ORLA85 protein). Capture antibodies were immobilized on the ORLA85 protein-modified SAW device surface, and Der f 1 was measured by detecting viscoelastic change induced by sandwich assay. In addition, differential method was employed to shorten the measurement time. It utilized a slope of the signal change as a sensor output instead of a signal shift that was used conventionally, and the measurement time was shortened by 10Â min from 30 min while maintaining the sensitivity. Also a stable sensor surface to pH change during the regeneration allowed repeated immunosensing of Der f 1 with high reproducibility—coefficient of variation of 5.7% in ten measurements.