Antibodies are the classical adaptor molecules of the immune system, linking recognition and killing of foreign pathogens. However, the recent discovery of a new property of the antibody molecule suggests a previously unexplored effector function of the immune system. All immunoglobulins, regardless of source or antigenic specificity, can catalyze the reaction between singlet (¹Δg) oxygen and water to give hydrogen peroxide (H₂O₂). Both the chemical and biological aspects of this pathway are being explored and intriguing new insights into how this pathway might have a role in immune defense are emerging.