Anandamide's effects can occur in either the central or peripheral nervous system. These distinct effects are mediated primarily by CB1 cannabinoid receptors in the central nervous system, and CB2 cannabinoid receptors in the periphery. The latter are mainly involved in functions of the immune system. Cannabinoid receptors were originally discovered as being sensitive to Δ9-tetrahydrocannabinol (Δ9-THC, commonly called THC), which is the primary psychoactive cannabinoid found in cannabis. The discovery of anandamide came from research into CB1 and CB2, as it was inevitable that a naturally occurring (endogenous) chemical would be found to affect these receptors.