PSK and PSP are known as immune modulators. Immune modulators have effects that can be attributed to how they essentially stimulate the body’s immune system, making it more active, resulting in a stronger immunity. 
Turkey Tail has been used in traditional medicine for centuries. In modern times, scientists have studied the mushroom to understand how it can fight fatigue.
They've found that a person's white blood cell count rises when taking these supplements, which indicates the mushroom is capable of actively stimulating a specific part of your immune system. With a rise in white blood cells, these supplements also stimulate macrophages, or "big eaters" that devour foreign cells and other substances. 
Having immune-modulating properties allows Turkey Tail also to have anti-fatigue properties, as fatigue at its root is an unhealthy immune system. 
A study conducted at the University of Toyama in Japan found that PSK was more effective than placebo in patients experiencing fatigue. This randomized, double-blind study tested two groups of men who were experiencing fatigue; one group received PSK daily for five weeks, while the other just received a simple sugar pill.
At the end of the five weeks, those who received PSK reported that their fatigue symptoms had decreased significantly. What's more, these reductions continued into the two weeks after treatment had ended.
In another study, scientists have found that PSP increases the body's production of interferon. Interferon helps cells communicate with one another, so when their numbers are high in your body you feel more energized. It also stimulates your lymphocytes, or white blood cells so that they can circulate throughout your body at a more frequent rate.