The humanized antibody efalizumab is currently the only T-cell directed biologic approved for the treatment of moderate-to-severe psoriasis by both American and European authorities. Binding to and blocking the function of the adhesion molecule leukocyte function associated antigen 1 (LFA-1), it is believed to interfere with T-cell activation in the lymph node, migration through the circulation into the skin, and re-activation in-loco, all of these representing central steps in the pathogenesis of psoriasis. A comprehensive clinical development program provided large and consistent evidence that efalizumab induces a major clinical benefit in psoriasis. Efalizumab rapidly and substantially improves psoriatic skin symptoms and leads to profound gain in quality of life. It allows safe and effective long-term control of psoriasis. Therefore, evidence-based treatment guidelines recommend its use in moderate to severe plaque-type psoriasis.