In this 2020 review it was concluded that vitamin C plays an intricate role on the immune system. It is crucial in maintaining the physical barrier of cells, especially the mucosal layer in the respiratory tract. It promotes collagen synthesis, thereby providing a protective barrier to cells.
Gombart, A. F., Pierre, A., & Maggini, S. (2020). A Review of Micronutrients and the Immune System-Working in Harmony to Reduce the Risk of Infection. Nutrients, 12(1), 236. https://doi.org/10.3390/nu12010236