Research points toward cannabinoids having an adaptive, immunomodulating effect, rather than just suppressing immune activity.

Some studies suggest that cannabinoids like THC and CBD are immunosuppressant, which can explain the relief experienced by medical cannabis users with autoimmune diseases and chronic inflammation. 
It gets even more complicated when we consider that the effects of cannabis are mediated primarily by the endocannabinoid system, which scientists believe interacts with all biological activity, including our immune system.
Endocannabinoids are produced on demand, travelling backwards across chemical synapses and modulating cell activity. This partly explains why the ECS has been termed a homeostatic regulator – continually working to maintain a state of biological balance.
The ECS regulates a plethora of physiological processes, including immune function and inflammation. Both CB1 and CB2 receptors can be found on immune cells, although there are between 10-100 times more CB2 receptors than CB1. Endocannabinoids act upon immune cells directly through the CB2 receptor.
CB2 receptor activation creates an anti-inflammatory effect and is therefore a therapeutic target for autoimmune disorders and neurodegenerative disease.1 However, any ECS immunosuppressant activity is thought to be transient, and can be overridden when necessary in the presence of infection.2

CANNABIS AND THE IMMUNE SYSTEMWhen we talk about cannabis, we’re dealing with upwards of 400 different molecules. These include the more frequently studied cannabinoids like THC and CBD, more than 100 other minor cannabinoids, dozens of terpenes, and a host of flavonoids – the combination of which varies according to the cannabis strain.

CBD, despite little binding affinity with cannabinoid receptors, is also considered to be immunosuppressant, reducing cytokine production3 and inhibiting T-cell function4.
But that’s only part of the story. A new wave of research and mounting anecdotal evidence points towards cannabinoids having an adaptive, immunomodulating effect, rather than just suppressing immune activity.

Sources
1. Caroline Turcotte, Marie-Renée Blanchet, Michel Laviolette, and Nicolas Flamand. The CB2 receptor and its role as a regulator of inflammation. Cellular and Molecular Life Sciences. 2016; 73(23): 4449–4470. doi: 10.1007/s00018-016-2300-4
2. Rupal Pandey, Khalida Mousawy, Mitzi Nagarkatti, and Prakash Nagarkatti. Endocannabinoids and immune regulation. Pharmacol Res. 2009 Aug; 60(2): 85–92, doi: 10.1016/j.phrs.2009.03.019