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CANNABIS is a double-edged sword: by dampening the immune system, it provides relief from inflammatory diseases, but this also increase the risk of infections. Now we know how it does this: its active ingredient targets a newly discovered type of cell that lowers the immune response.

at the University of South Carolina School of Medicine and colleagues injected the main active ingredient of cannabis, delta-9-tetrahydrocannabinol (THC), into mice. THC activated two types of cannabinoid receptor on immune cells, called CB1 and CB2. Activation of these receptors led to a “massive mobilisation” of myeloid-derived suppressor cells (MDSCs), which play a crucial role in lowering the immune system response back down to normal levels (European Journal of Immunology, ).

The discovery offers a possible explanation of why cannabis smokers have a higher risk of getting infections, says Nagarkatti. It may also mean THC could be used when there is a need to suppress the immune system – after an organ transplant, for example.