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