Wim Hof, also known as The Iceman, is a Dutch extreme athlete noted for his ability to withstand freezing temperatures. This is a method he developed, that has some evidence that it may help our immune system. It involves three aspects, a breathing technique, meditation process and gradual exposure to cold.
The breathing technique appears to help alkalise the body by reducing CO2 levels and increasing oxygen consumption. Our body being in an alkaline state may help our immune system to respond better to bacteria and viruses. What I like about this breathing technique is that you can feel how it gets the air down low into your lungs and it makes your chest cage and diaphragm move well.
Meditation: to reduce hyperactivity of our sympathetic nervous system our flight fright system of our body. Meditation can assist in reducing our stress hormone. High stress hormone levels reduce the effectiveness of our immune response.
Gradual exposure to cold. Ice baths may not look very attractive to you, however again there is some evidence to their health benefits and may interest you.