We present an all-fiber Mach-Zehnder modal interferometer based on a special photonic crystal fiber taper and its performance as a transducer for biological sensing. The sensor showed refractive index sensitivity around 722.3 nm/RIU for measurements between 1.30864 and 1.32014 RIU We investigated the interferometer as a transducer for an immunosensor system after immobilization of an antigen over the activated taper waist, where it detected the interaction between antigen and antibody achieving a record detection limit of 125 pg/ml of antibody concentration. This extra low detection limit allows the modal interferometer to be used in biosensing applications, such as disease diagnosis, or in the food industry.