Apples are a rich source of phytonutrients that are important for your immune system, like vitamin C and antioxidants.

Vitamin C helps your body to produce antibodies and your immune cells to travel to sites of infection, while antioxidants can reduce excess inflammation, helping you bounce back faster if you get sick.

More than that, the fruit is particularly good for maintaining a healthy gut.
Given that 70% of your immune cells live in your gut, a healthy gut microbiome is an important part of supporting your immune system.

Apples support this system in two ways. First, just one apple contains 100 million microbes, which have been shown to contribute to your gut microbiome, helping it to break down food and make phytonutrients available to the body to use.
Second, an average apple contains around 4g of fibre – this is a fuel source for the good bacteria in your gut, which help regulate immune response.

In addition to their culinary versatility and numerous colors and flavors, apples are an exceptionally healthy fruit with many research-backed benefits. 

Source: Guardian

https://www.theguardian.com/lifeandstyle/2024/nov/18/is-it-true-that-an-apple-a-day-keeps-the-doctor-away