Alemtuzumab, formally known as Campath-1H, is a humanized monoclonal antibody directed against CD52, a protein on the surface of lymphocytes and monocytes with unknown function. A single dose of alemtuzumab leads to a rapid, profound and prolonged lymphopenia. A Phase II trial has shown that alemtuzumab reduces the risk of relapse and accumulation of disability by over 70% compared with interferon beta in patients with early relapsing-remitting multiple sclerosis (MS). Alemtuzumab has been used in Cambridge as an experimental treatment for MS since 1991. In this review we summarize our experience; describing how this prototypical, "bench-to-bedside" therapy continues to inform basic science, revealing aspects of the pathogenesis of MS and lymphopeniaassociated autoimmunity.