The invention features peptides containing domain 4 of the Streptococcus intermedius intermedilysin (ILY) protein and fragments thereof, and the use of these peptides to sensitize cancer cells to antibody based anticancer treatments. CD59 receptor activity has been associated with a decreased sensitivity to therapeutic antibodies. Desirably, administration of ILY domain 4 polypeptides is sufficient to inhibit CD59 receptor activity while avoiding the general toxicity associated with full length ILY.