Editing Erythema multiforme (section)

=== Herpes simplex virus ===
Herpes simplex virus (HSV) is by far the most frequent cause of erythema multiforme. HSV DNA is also detected in nearly half of patients with [[Idiopathic disease|idiopathic]] EM, suggesting that the true frequency is even higher than traditionally thought.<ref name=":1" /> Because of this frequency, more is understood about EM caused by HSV than any other type.<ref name=":0" />

100% of HSV-associated EM cases are found to have DNA for viral-type [[DNA polymerase]] (Pol) gene within keratinocytes. Despite this association, an active and infectious form of HSV has never been found in a patient suffering from EM, indicating that it is exclusively a post-viral hypersensitivity syndrome in these patients, rather than a symptom of the virus itself. The viral DNA polymerase protein is synthesized by [[Basal keratinocyte|basal keratinocytes]], which then present the protein or fragments of its DNA to immune cells via [[MHC class I|MHC I]]. The resulting immune reaction results in the creation of [[Cytotoxic T cell|CD8+ T-cells]] specific to HSV, which then induce a strong inflammatory response, recruiting other immune cells to places where infected keratinocytes are identified. Keratinocyte production of Pol protein only lasts for a few days, however, so most HSV-related cases of erythema multiforme do not become chronic or recurrent.<ref name=":0" /> However, recurrence is a known possibility with this type of EM, potentially justifying the use of preventative antiviral therapy.<ref name=":1" /><ref name=":2" />