Lemon (Citrus limon) is a natural immune system booster. It has great detoxifying and antiseptic benefit. Due to its uplifting effect it can be a great anti depressant.
"This oil with the refreshing scent stimulates the liver, has a gentle, calming effect (despite its high terpene hydrocarbon content), and is perhaps the most effective oil for disinfecting room air using a diffusor"
Source: Kurt Schnaubelt, Ph. D.: Advanced Aromatherapy
Jennie Harding: Secrets of Aromatherapy