Vitamin C and immune system

Several cells of the immune system can indeed accumulate vitamin C and need the vitamin to perform their task, especially phagocytes and t-cells. Thus a vitamin C deficiency results in a reduced resistance against certain pathogens whilst a higher supply enhances several immune system parameters (framework).

https://www.ncbi.nlm.nih.gov/pubmed/19263912

Vitamin C concentrations in the plasma and leukocytes rapidly decline during infections and stress. Supplementation of vitamin C was found to improve components of the human immune system such as antimicrobial and natural killer cell activities, lymphocyte proliferation (rapid increase in numbers), chemotaxis (movement of an organism in response to a chemical stimulus), and delayed-type hypersensitivity (inflammatory reactions initiated by mononuclear leukocytes).

https://www.ncbi.nlm.nih.gov/pubmed/16373990