Multiple cell types and their factors and cytokines are involved in regulating the immune response in inflammatory skin diseases. Stimulatory and inhibitory factors interact to determine whether the immune response is regulated up or down. Normally, stimulatory signals are counteracted by inhibitory signals to prevent an immune reaction from being initiated. However, exogenous antigens and irritants or endogenous factors and altered immunogenic self-peptides can upset this balance. When that occurs, T cells are activated and an inflammatory skin reaction develops. Lymphokines released from such activated T cells can modify the phenotype and function of normal keratinocytes. They can induce the expression of adhesion molecules and receptors involved in antigen presentation. Furthermore, they can also stimulate keratinocyte proliferation. This may be important in development of the hyperplasia seen in inflammatory skin diseases, especially in psoriasis. Cytokines released from the activated keratinocytes can both stimulate and attract T cells to the epidermis and thereby continue the ongoing immune reaction.