The present invention provides reagents, methods and systems for predictingthe cytotoxic activity of an antibody or variant thereof comprising: determininga binding affinity of the antibody or variant thereof to a Fc activating receptor；determining a binding affinity of the antibody or variant thereof to a Fc inhibitoryreceptor, and calculating the ratio of said activating binding affinity to saidinhibitory binding affinity (A/I ratio) , wherein the magnitude of said ratiois an indication of the cytotoxic activity of the antibody or variant thereof. The present invention also provides purified modified antibodies having alteredA/I ratios as compared to the unmodified antibodies.
