If cannabis is ingested it doesn't have any effect on the immune system as far as I've read/heard/experienced.

However if smoked it can have a substatial effect on the immune system in some people if they smoke often, this isn't because of the drug itself, it's because of the actual smoke.

However people who participate in regular excercise, eat healthy, live a healthy lifestlye and smoke cannabis will see largely no harm to their immune system.