Wim Hof is great. I encourage everyone to read his books.

Here is an article written by the researchers summing up their study (which is linked in the OP):

With simple techniques such as breathing exercises, meditation and repeated exposure to cold, you can activate the autonomic nervous system and inhibit the response of your immune system. Researchers from Radboudumc provide the first scientific evidence for this in an article that was published yesterday in the leading journal PNAS.

A well-functioning immune system protects our body against pathogens. But sometimes the immune system overreacts and makes too many inflammatory proteins. This can lead to autoimmune diseases such as rheumatism. Among other things, our immune system is under the control of the autonomic nervous system, which is involved in the 'fight-or-flight response'. Both the immune system and the autonomic nervous system have long been thought not to be consciously influenced. Experiments by Intensive Care researcher Matthijs Kox and Professor of Experimental Intensive Care Medicine Peter Pickkers now show that it is possible with certain techniques. However, the researchers emphasize that it has not yet been determined whether these techniques could be effective in patients. Further research is needed for this.

Training

'Iceman' Wim Hof ​​trained twelve healthy young men for ten days with a few specific techniques. Part of the training took place in Poland, where the volunteers learned meditation and breathing exercises and walked through the snow in shorts. Back in the Netherlands, the scientists gave the trained men and twelve healthy untrained men an injection of endotoxin. This is a component of the cell wall of bacteria and causes a reaction of the immune system. Pickkers: “By administering a dead bacterial part, we are actually fooling the body. The immune system reacts as if live bacteria are invading and makes inflammatory proteins. As a result, the test subjects develop flu symptoms such as fever and headache. That way we can study the immune system in humans.”

Stress Hormone

Kox: “The trained men produced more of the hormone adrenaline as a result of the techniques learned.” Adrenaline is a stress hormone released by increased activity of the sympathetic nervous system and suppresses the immune response. “We did indeed see fewer inflammatory proteins in the trained men and they suffered much less from flu symptoms,” says Kox.

Earlier Iceman Research

Iceman Wim Hof ​​is known for his various cold records, which he believes were possible by influencing his autonomic nervous system. In 2011, researchers at Radboudumc looked at Hof's body's response to an endotoxin injection while he practiced techniques he had developed himself. He was found to produce half as many inflammatory proteins as healthy volunteers who did not use the techniques. In addition, he showed virtually no flu symptoms. The results were so remarkable that the scientists decided to do further research.