There is no conclusive evidence that cannabis impairs immune function to any significant extent. The few studies that have suggested that cannabis has an adverse effect on the immune system have not been replicated. So, the evidence here is not the most reliable.
All of the following sites should be treated with a degree of caution, because long-term analysis of the effect of cannabis on the immune system has never really been carried out, and so these sites cannot be as authoritative as they may claim to be.