An optical fiber immunosensor based on long period gratings (LPGs) made by periodic laser ablation is presented in this work. In order to be able to modify the effective index of the fundamental and the cladding modes, a cladding-etched SMF (CE-SMF) was coated with tin dioxide and after that the grating was created by periodic ablation of the overlay. Type G immunoglobulins (IgGs) were attached to (3-glycidyloxypropyl) trimethoxysilane (GPTMS) overlay. Samples with anti-IgGs concentrations ranging from 1 mg/L to 100 mg/L have been studied and a maximum sensitivity of 1.2 nm/(mg/L) was obtained. There is still room for the improvement of this optical platform.