CB1 and CB2 receptors
The two receptors through which cannabinoids, whether from the plant or from the body, act.
CB1 and CB2 are the receptors of the endocannabinoid system. Both belong to the large family of G protein-coupled receptors that sit in the cell membrane and translate a signal from outside into a response within.
CB1 was identified first, in 1990, and is abundant in the brain — cortex, basal ganglia, cerebellum, hippocampus. It is notably scarce in the brainstem, which is the accepted explanation for why cannabis does not suppress breathing the way opioids can.
CB2 was characterised three years later and sits mainly in the immune system: spleen, tonsils, circulating immune cells, and microglia in the brain when inflammation is present. That division is a simplification, but it holds well enough to explain why some compounds act on mood and perception while others act mainly on inflammation.