Mistletoe is a plant that has been used for centuries for various medical conditions. In recent years, it has gained attention for its anti-cancer properties. While mistletoe likely has limited efficacy as a stand alone treatment, when appropriately combined with other therapies it may have several important cancer-fighting benefits:

Increased immunity: in a normal immune system, cells that are aged, diseased, or cancerous cells are targeted and destroyed. When the immune system becomes weak or compromised, cancer cells can evade immune destruction, gaining the ability to grow, spread, and strengthen in number. Mistletoe extract contains several compounds that can help the immune system. It can specifically boost the production of a specific type of immune cells, called the natural killer cells, which play a major role in recognizing and destroying cells. 
Direct anti-cancer activity: Mistletoe extract has been shown to have direct anti-cancer activity. It can inhibit the growth and spread of cancer cells, and it also contains compounds that are responsible for apoptosis (programmed cell death) in cancer cells.

Decreased side effects from other cancer treatments: some studies show that mistletoe can decrease the nausea and vomiting from chemotherapy and also improve overall quality of life. 

Increased survival: there is promising evidence that overall survival may be improved in patients treated with mistletoe.