Psoriasis is thought to be the result of T-cell activation initiated by unidentified antigens. These activated T-cells release cytokines like tumor necrosis factor alpha (TNF-α) that lead to keratinocyte proliferation. This hyper-proliferative response decreases epidermal transit time from the normal 28 days to 2–4 days, ultimately leading to the erythematous scaly plaques that characterize the disease.