Functions of vitamin C in humans:[1]

collagen biosynthesis and repair
carnitine synthesis
redox-reactions
production of adrenal steroids and catecholamines
metabolism of amino acids and cholesterol
iron absorption

Immune System

In order to protect themselves from oxidative damage, neutrophils accumulate millimolar (mM) concentrations of vitamin C, resulting in improved cellular motility and migration in response of chemotactic stimuli and, subsequently, in enhanced phagocytosis of microbes and generation of reactive oxygen species[1]

Moreover, vitamin C application to monocytes derived from human whole blood diminished secretion of pro-inflammatory cytokines such as IL-6 and tumor necrosis factor (TNF)-α. In addition, vitamin C treatment was shown to stimulate and enhance phagocytosis and clearance of macrophages in vitro. Taken together, these findings underline the important role of vitamin C in host defense against pathogens.[1]

In addition, the observations that vitamin C concentrations in immune cells such as leukocytes are 10- to 100-fold higher than those measured in the plasma and the fact that these cells accumulate vitamin C against a concentration gradient further underline the immunological importance of vitamin C.[1]

Antimicrobial

As early as the 1930s, vitamin C has been known for its antimicrobial effects directed against Mycobacterium tuberculosis, the infectious agents of human tuberculosis.[1]

Initially it was hypothesized that the antimicrobial properties of vitamin C were due to its pH lowering effect [93]. Another study, however, could prove potent antimicrobial effects of vitamin C directed against group A hemolytic streptococci, even in a nearly pH-neutral environment.[1]

Further studies assessed the antibacterial effects of vitamin C against distinct bacterial (opportunistic) pathogens, in more detail, applying microdilution assays.[1]

Vitamin C application at low concentration (0.15 mg/mL) was shown to inhibit the growth of Staphylococcus aureus.[1]

Vitamin C could even effectively counteract biofilm formation by methicillin-resistant S. aureus (MRSA), displaying low-level resistance to vitamin C (8 to 16 μg/mL)[1]

The antibacterial effects of vitamin C might be both, bacterial strain and concentration dependent.[1]

Notably, the co-administration of vitamin C could sufficiently enhance the antibacterial effects of other agents such as epigallocatechin gallate directed even against multidrug-resistant bacterial species such as MRSA, which also held true for vitamin C in combination with deferoxamine against Gram-positive cocci, such as S. aureus and S. epidermidis, as well as against Gram-negative bacilli, including E. coli, Klebsiella pneumoniae and Proteus mirabilis. Synergistic antibacterial effects could also be observed upon co-administration of vitamin C and quercetin, whereas the combination of vitamin C with natural extracts such as pomegranate rind extracts and white tea resulted in enhanced anti-S. aureus properties of the latter.[1]

Surprisingly, the co-administration of vitamin C and the anti-malaria drug artemether reduced anti-parasitic activity in Plasmodium berghei malaria infected mice as compared to artemether application alone.[1]

Vitamin C is chemically L-ascorbic acid. The L-ascorbic acid is very labile to heat and oxygen. While in storage, L-ascorbic acid is oxidized to dehydro-L-ascorbic acid, which undergoes spontaneous hydration to form di-keto-L-gulonic acid. This form (di-keto-L-gulonic acid) of vitamin C is biologically inactive, therefore, vitamin C activity is slowly decreases while its stored.

[1] https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6798581/