Psoriasis is a disease characterized by scaly skin lesions secondary to keratinocyte hyperplasia. The presence of active T cells in the lesions, experimental observations on disease transfer, and therapeutic efficacy of specific immunosuppressive drugs have led to the identification of the activated T lymphocyte as the primary factor for keratinocyte stimulation. Understanding the pathways of pathogenesis is fundamental in evolving therapies for intervention at different points in the pathogenic model and for curtailing the process. Advances in biotechnological methods have helped to create designer molecules and proteins that specifically recognize target receptors and chemicals that modify their actions. These drugs, termed "biologic response modifiers," are now being studied as specific immunosuppressive agents producing different T-cell and cytokine effects in psoriasis.