Summary
Immune cells require large amounts of vitamin C to function well. An increased turnover of vitamin C in the body due to, for instance, stress and illness can induce vitamin C deficiency when its intake is too low. Several immune processes are influenced by vitamin C. Consequently, a sufficient level of vitamin C is necessary for a well-functioning immune system. According to a study with vitamin C-deficient mice, supplementation of high levels of vitamin C significantly increased the number of immune cells. More details can be found in the review article.
References
Ang et al.