This is due to local skin memory T-cells, which remain in the original sensitization site. In a similar fashion, cytotoxic T lymphocytes patrol an area of skin and play an important role in controlling both the reactivation of viruses (such as the "cold sore" virus) and in limiting its replication when reactivated.[17] Memory response, or "Retest Reactivity", usually takes 2 to 3 days after coming in contact with the allergen, and can persist for 2 to 4 weeks.