So, there’s a lot of mixed messaging when it comes to mushrooms effects on the immune system. The short answer is that it does seem that some mushrooms can have a positive impact on the bodies immune response. But, there is a large variation in which mushrooms are beneficial and to top it off, the dosages and the particular compounds in mushrooms that actually have these effects.
The most common issue that we’ve found with mushroom supplements is that there simply isn't enough of the compounds you need in the doses that they’re in. Typically, unrefined mushroom extracts, which make up the majority of these supplements that claim to boost your immune system.
There are some high yield extracts which means it is doable in capsule form, just most of the industry is unfortunately unscrupulous. The reason it can be done with high yield extracts is there are a couple of notable compounds like the most well researched, b-Glucan. 
It’s also worth noting that most of the studies that have been done on more severe conditions and in relation to radiation have been done on mice, not on humans, so any claims around Mushrooms and this sort of immunity should be taken with a pinch of salt. Although the studies are of course interesting.
Mushrooms and Immunity: The studies
As we mentioned a lot of the studies on more serious conditions have been done on mice, and a lot of this comes down to their content of B Glucan as we mentioned in the intro. Studies have shown that this compound can increase the effectiveness of what’s known as natural killer cells (NK) and these are used to break down tumours.  On top of that studies have shown mushrooms can increase the creation of T and B cells which are also part of the bodies immune response. 
Studies have shown that mushrooms can increase mouse survival and protect against radiation issues, mostly using b-glucan from Reishi, although these studies are largely on mice. 
In terms of more normal immune function, which is what most people look at mushroom supplements for, the most commonly cited study was done by the University of Florida.
The study took healthy adults between the ages of 21 and 41. The participants were provided with a four-week supply of dry shiitake mushrooms and instructed to consume a 4 ounce serving of mushrooms each day. Yes, 4 ounce. Which is a lot more than we often get in most mushroom supplements. Some mushroom powders would of course have enough. 
Before and after the experiment blood tests were conducted to check the participants' immune systems. The results were positive. With the researchers observing better functioning T-cells and a reduction in inflammatory proteins. This did of course show, that it did indeed improve immunity.
Which Mushrooms Are Best For Immunity?
Shitake - Unsurprisingly we ranked Shitake in here, seeing as we referenced its study earlier. There are of course a lot of other studies, and it's quite well backed. You need quite a lot of it, or high quality extract.
Reishi - Another one of the more well studied mushrooms for immune response. A lot of Reishi mushrooms effect comes down to it’s polysaccharides and triterpenes. And one in particular b-glucans, which have shown some noteworthy effects in improving the bodies immune responses, in both healthy and unhealthy participants. 
Cordyceps - Cordyceps extracts (particularly Cordycepin) have been found to increase IL-1 and IL-2 production. Both of which are important for immune cell activation. They have also been shown to enhance phagocytosis (the bodies ability to purge forieng organisms such as unwanted bacteria) and increase the effect of NK cells. 
Are There Any Good Mushroom Supplements for Immunity?
There are a couple, generally we recommend you stick to a powder, as we mentioned most of the effective studies have been done on ounces rather than the couple of grams you get in capsules. Currenlty, the only capsule supplement we can recommend is Nootrum, as they actually use the correct high b glucan extracts you'd need to get the effects. But, it's a bit on the higher side price point wise.
Conclusion: Do Mushrooms Boost Immunity?
The conclusion is that it does look like mushrooms can in fact improve immunity. The catch is that most supplements don't have nearly enough of the required compounds to be effective. We generally recommend Nootrum, but most mushroom supplements are not going to do anything.
1 - https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7770584/
2 - https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4684115/
3 - https://www.ncbi.nlm.nih.gov/pmc/articles/PMC1160565/
4 - https://www.ncbi.nlm.nih.gov/pmc/articles/PMC9504980/
5 - https://pubmed.ncbi.nlm.nih.gov/25866155/
6 - https://www.ncbi.nlm.nih.gov/pmc/articles/PMC10480226/
7 - https://pubmed.ncbi.nlm.nih.gov/1392475/