Several cells in the brain and other organs contain specific protein receptors that recognize THC and some other cannabinoids and trigger cell responses. Other cannabinoids do not bind to these cannabinoid receptors and exert their effects by other ways. The discovery of specific cannabinoid receptors prompted the search for putative naturally-occurring chemicals that interact with the receptors, the endocannabinoids. There are at least two cannabinoid receptor types, CB1 receptors, and CB2 receptors. CB1 receptors are found in high concentrations within the brain and spinal cord. They are also present in certain peripheral cells and tissues (some neurons, some endocrine glands, leukocytes, spleen, heart and parts of the reproductive, urinary and gastrointestinal tracts). CB2 receptors are expressed primarily by immune cells und tissues (leukocytes, spleen and tonsils), but are also found in the brain.