Keratinocytes and dermal fibroblasts play an important role in the immune response of the skin.  In response to irritating chemicals or UV radiation (pro-inflammatory stimuli), keratinocytes can release a vast array of cytokines. These cytokines are thought to help engage immune cells to the site of irritation.  Cytokines released by the keratinocytes include TNFa, IL-1a, IL-1b, IL-3, IL-6, IL-7, IL-8, IL-10, IL-18, and IL-1RA.  In addition, prostaglandins are synthesized in a variety of cells from arachidonic acid.  Arachiconic acid can be released from membrane phospholipids via phospholipase A₂, and is then committed to form prostaglandins via the action of cyclooxygenase.  PGE₂ has been shown to be involved in the inflammatory pathway of the skin and is commonly used as a marker to show the efficacy of anti-inflammatory materials. Typical endpoints for this assay are the measurement of IL-1a and PGE₂, although changes in TNFa and NF-KB activation can also be measured.