Participants in this study will be randomly assigned to receive either alefacept or a placebo. A placebo looks just like the study drug but contains no active medication. Placebos help study doctors find out if the effects of the study drug are due to the drug itself or just because of being in the study. This type of study design helps measure the true effectiveness of a treatment. Participants have a 1 out of 2 chance of receiving alefacept and a 1 out of 2 chance of receiving placebo. Neither participants nor the study doctors will know which group participants have been assigned to.