From the Guidelines

Enhancing the immune system can be achieved through a combination of a healthy and balanced diet, adequate nutrition, and specific supplements.

A diet high in fiber, unsaturated fatty acids, and polyphenols, and low in saturated fats, sodium content, and processed carbohydrates, such as the Mediterranean diet, can help support immune function 1.
Probiotics, prebiotics, and certain supplements like vitamin D, zinc, and vitamin E can also play a role in enhancing immune function 1.

Consuming at least five portions of fruit and vegetables per day can help reduce infection risk 1.
Limiting alcohol intake to no more than 2 units per day and avoiding binge drinking can also help support immune function 1.

Vitamin D supplementation (2000 IU/day) can help support immune defense against the common cold, especially for individuals who are deficient or insufficient in vitamin D 1.
Zinc supplements (75 mg/day) can help reduce the duration of cold symptoms 1.
Probiotic ingestion (daily doses of Lactobacillus and Bifidobacterium species) can help reduce the incidence of respiratory illness 1.
Vitamin E supplementation (200 IU/day) can help improve T cell-mediated immune functions in older adults 1.

From the Research

To enhance the immune system, several strategies can be employed, including:

Consuming adequate amounts of vitamin C and zinc, as these nutrients play important roles in immune function and the modulation of host resistance to infectious agents 2
Engaging in regular physical activity, as long-term physical activity is beneficial in improving the immune system and preventing various infections 3
Eating a balanced and nutritious diet that includes a variety of fruits, vegetables, whole grains, proteins, and probiotics, as these foods provide essential nutrients and bioactive compounds that can boost the immune system 3, 4
Considering supplementation with probiotics, bovine colostrum, plant-derived products, and functional foods, as these may provide additional benefits in improving the immune system 3
Maintaining adequate levels of micronutrients such as vitamins A, D, and E, and minerals like zinc, iron, and selenium, as deficiencies in these nutrients can impair immune function 4, 5

Some key nutrients and compounds that can help enhance the immune system include:

Vitamin C, which contributes to maintaining the redox integrity of cells and protects them against reactive oxygen species generated during the respiratory burst and in the inflammatory response 2
Zinc, which is essential for the functioning of cellular mediators of innate immunity such as phagocytosis, natural killer cell activity, and the generation of oxidative burst 2
Probiotics, which can modify the gut microbiota and enhance immune function in laboratory animals and may do so in human subjects 3, 5
Polyphenols, terpenoids, β-glucans, and other bioactive phytoconstituents found in plants and fungi, which have potent immunomodulatory properties and can augment the immune response 4
Melatonin, a multifunctional molecule with proven anti-inflammatory and immunomodulatory properties, which can be found in plant sources 4

A well-balanced diet and a healthy lifestyle are crucial for maintaining a strong immune system, as:

Undernutrition decreases immune defences, making an individual more susceptible to infection 5
The immune response to an infection can itself impair nutritional status and alter body composition 5
Regular physical activity and a balanced diet can help prevent various infections and improve overall health and well-being 3