An immunosensor using a long-period grating (LPG) was used for sensitive detection of antibody−antigen reactions. Goat anti-human IgG (antibody) was immobilized on the surface of the LPG, and detection of specific antibody−antigen binding was investigated. This sensor operates using total internal reflection where an evanescent field interacts with bound antibody immobilized over the grating region. The reaction between antibody and antigen altered the LPG transmission spectrum and was monitored in real time as a change in refractive index, thereby eliminating the need for labeling antigen molecules. Human IgG binding was observed to be concentration dependent over a range of 2−100 μg mL^-1, and equilibrium bound antigen levels could be attained in ∼5 min using an initial rate determination. Binding specificity was confirmed using human interleukin-2 and bovine serum albumin as controls, and nonspecific adsorption of proteins did not significantly interfere with detection of binding. Antigen detection in a heterogeneous protein mixture and in crude cell lysate from Escherichia coli was also confirmed. Moreover, regeneration of the LPG surface via diethlyamine treatment resulted in ∼80% removal of bound antigen. Subsequently, fibers reexposed to antigen retained greater than 85% of the initial signal after five consecutive regeneration cycles.