Does marijuana weaken the immune system?

Possibly: Because it can negatively affect the quality of sleep it could via that mechanism. Chronic inhalation of smoke can compromise the respiratory system and therefore immunity at least of that portion of your system. Many do not recognize the impact of chronic cannabis abuse because they do not stop it long enough to establish a baseline. It is in the blood for two weeks after use.

Really challenging: This is far too complex to even scratch the surface, as not too much is known regarding the full effects of marijuana. But, there is some evidence of benefits on spasticity in ms, but also, theoretical issues with interaction with ms drugs, such as gilenya (fingolimod). Lung function can be affected, and this would disturb the first line immune system indirectly. All the answers not known yet.