A ternary system consisting of a protein, catechin (either + or - epimer), and Tb(III) in suitable aqueous buffer medium at physiological pH (= 6.8) has been shown to exhibit highly efficient "antenna effect". Steady state and time-resolved emission studies of each component in the binary complexes (protein with Tb(III) and (+)- or (-)-catechin with Tb(III)) and the ternary systems along with the molecular docking studies reveal that the efficient sensitization could be ascribed to the effective shielding of microenvironment of Tb(III) from O-H oscillator and increased Tb-C (+/-) interaction in the ternary systems in aqueous medium. The ternary system exhibits protein-mediated efficient antenna effect in D2O medium due to synergistic ET from both the lowest pi pi* triplet state of Trp residue in protein and that of catechin apart from protection of the Tb(III) environment from matrix vibration. The simple system consisting of (+)- or (-)-catechin and Tb(III) in D2O buffer at pH 6.8 has been prescribed to be a useful biosensor.