Which nutrient is known to improve immunity?

Vitamin A is known to play a significant role in supporting and improving immunity. It is essential for maintaining the integrity and function of epithelial tissues, which act as a barrier against pathogens. Moreover, Vitamin A is vital for the development and differentiation of immune cells, particularly lymphocytes. It helps in the production of antibodies and enhances the activity of white blood cells, which are crucial components in the body's defense against infections.

In addition to its role in immune function, Vitamin A is also involved in the vision process, cellular communication, and the maintenance of healthy skin and mucous membranes. Consuming adequate amounts of Vitamin A through diet, primarily from sources such as carrots, sweet potatoes, spinach, and other leafy greens, can contribute to a stronger immune system.

The other options do not have the same direct and well-established relationship with improving immunity. Sodium is primarily associated with maintaining fluid balance and not specifically linked to immune enhancement. Vitamin D does support immune function, but in this context, Vitamin A is better known for its significant role. Caffeine, while it may provide short-term enhancements in alertness and physical performance, does not directly contribute to immune function in the same way that Vitamin A does.