Abstract: This invention relates to methods of preventing or reducing the severity of psoriasis. In one embodiment, the method involves administering to the individual a peptide having substantially the sequence of a non-conserved region sequence of a T cell receptor, present on the surface of T cells mediating psoriasis or a fragment thereof, wherein the peptide or fragment is capable of causing an effect on the immune system to regulate the T cells. In particular, the T cell receptor has the V&bgr; region-V&bgr;3, V&bgr;13.1 or V&bgr;17. In another embodiment, the method involves gene therapy. The invention also relates to methods of diagnosing psoriasis by determining the presence of psoriasis predominant T cell receptors.