Soursop is high in vitamin C, an antioxidant known to boost immune health. The vitamin strengthens your immune system, improving its ability to defend against pathogens. It also promotes the destruction of free radicals, which can help to protect your skin and cells from environmental oxidative damage. One whole soursop fruit contains 215% of your recommended daily allowance of vitamin C. 
Soursop (the fruit and the leaves) contains many other antioxidants, including phytosterols, tannins, and flavonoids. Antioxidants play a role in your overall health and may help to protect against a variety of health conditions.