Immunostab is a biologically active supplement which contains 650 mg Vitamin C and 22 mg Zinc Citrate.

Vitamin C

Vitamin C or ascorbic acid (AA) was first isolated in 1923 by Hungarian biochemist and Nobel laureate Szent-Gyorgyi and synthesized by Howarth and Hirst. [1] It exists in reduced [ascorbate] and oxidized forms as dehydroascorbic acid which are easily inter-convertible and biologically active thus it acts as important antioxidant. Vitamin C is easily oxidized acid and destroyed by oxygen, alkali and high temperature. Most of the plant and animal species have the ability to synthesize vitamin C from glucose and galactose through uronic acid pathway but man and other primates cannot do so because of deficiency of enzyme gulonolactone oxidase required for it’s biosynthesis. Deficiency of this enzyme is a result of a mutation which occurred approximately 40 million years ago. [2]
The body requires vitamin C for normal physiological functions. It helps in the synthesis and metabolism of tyrosine, folic acid and tryptophan, hydroxylation of glycine, proline, lysine carnitine and catecholamine. It facilitates the conversion of cholesterol into bile acids and hence lowers blood cholesterol levels. It also increases the absorption of iron in the gut by reducing ferric to ferrous state. As an antioxidant, it protects the body from various deleterious effects of free radicals, pollutants and toxins. Vitamin C have been widely used in the treatment and prevention of a large number of disorders like diabetes, atherosclerosis, common cold, cataracts, glaucoma, macular degeneration, stroke, heart diseases, cancer and so on. [3]
Vitamin C is found in citrus fruits, green peppers, red peppers, strawberries, tomatoes, broccoli, brussels sprouts, turnip, Indian gooseberry and other leafy vegetables. It’s consumed either with diet or dietary supplements, is absorbed by the epithelial cells of the small intestine by SVCT1 or, subsequently diffuses into the surrounding capillaries and then the circulatory system [4]
Apart from the well accepted role of vitamin C in the prevention of scurvy, the most widely known health beneficial effect of AA is in the prevention and relief of common cold. High dose of vitamin C and suggested that ingestion of 1–3 g of AA effectively prevents/ameliorates common cold. [5] Vitamin C inhibits the excessive activation of the immune system to prevent tissue damage. It also supports antibacterial activity, stimulates natural killer (NK) cells and differentiation of Th0 subset into Th1 subset. In addition, vitamin C also modulates synthesis of proinflammatory cytokines, or expression of adhesive molecules. [6]
The potential benefit of vitamin C supplementation has been fueled in part by a considerable body of epidemiological literature suggesting a positive association between vitamin C status and health. Thus, several large cohort studies have shown an inverse relationship between plasma vitamin C status and risk of CVD and/or all-cause mortality [7]

Zinc Citrate

The water-soluble zinc salts gluconate, sulfate, and acetate are commonly used as supplements in tablet form to prevent zinc deficiency and to treat diarrhea in children in combination with oral rehydration. Zinc citrate is an alternative compound with high zinc content, slightly soluble in water․ [8]
Zinc is an essential trace element that has a critical role in maintaining structural and catalytic functions of >200 enzymes involved in major metabolic pathways, including nucleic acid metabolism, protein synthesis, and cell division. Although it remains difficult to define zinc status, zinc deficiency appears to be common among children in many developing countries, negatively affecting physical growth, immune competence, neural development, and reproductive outcomes, and increasing morbidity and mortality. The WHO considers zinc deficiency to be a major contributor to the burden of disease in developing countries, especially in those with a high mortality rate [9]. Several factors contribute to the development of zinc deficiency, including increased requirements at certain stages of the life cycle, malabsorption, impaired utilization, and increased losses attributable to repeated diarrhea. However, most often the primary cause of zinc deficiency is inadequate dietary zinc intake and low bioavailability of zinc attributable to the consumption of plant-based diets that are high in phytic acid, thus inhibiting zinc absorption [10].
By comparing zinc citrate, gluconate, and zinc oxide, zinc citrate is as well absorbed by healthy adults as zinc gluconate. Zinc citrate was shown to have an absorption rate of 61.3%, while zinc gluconate and zinc oxide had absorption rates of 60.9% and 49.9%, respectively. Also, the use of zinc citrate could provide an alternative to the more expensive form (zinc gluconate) or forms that give a strongly metallic, bitter, and astringent taste (zinc sulphate and acetate), which would need to be masked in a supplement.  [11]
In summary researches indicate that zinc is as well absorbed from zinc citrate as from zinc gluconate and that zinc citrate should be as effective as zinc gluconate in the prevention of zinc deficiency and presumably also in the treatment of diarrhea. Its higher zinc content, good sensory properties, and lower price make it an attractive alternative to gluconate and other water-soluble zinc compounds. [8]

References
[1] Haworth WN, Hirst EL. Synthesis of ascorbic acid. J Soc Chem Ind (Lond) 1933;52:645–647.
[2] Nishikimi M, Fukuyama R, Minoshima S, Shimizu N, Yagi K. Cloning and chromosomal mapping of the human nonfunctional gene for l-gulono-gamma-lactone oxidase, the enzyme for l-ascorbic acid biosynthesis missing in man. J Biol Chem. 1994;269:13685–13688.
[3] Dunitz JD. Linus Carl Pauling. 28 February 1901–19 August 1994. Biographical Memoirs of Fellows of the Royal Society. 1996;42:316–8.
[4] Malo C, Wilson JX. Glucose modulates vitamin C transport in adult human small intestinal brush border membrane vesicles. J Nutr. 2000;130:63–69.
[5] Elwood PC, Lee HP, St Leger AS, Baird M, Howard AN. A randomized controlled trial of vitamin C in the prevention and amelioration of the common cold. Br J Prev Soc Med. 1976;30(3):193–196.
[6] Holmannová D, Koláčková M, Krejsek J. Vitamin C and its physiological role with respect to the components of the immune system. Vnitr Lek. 2012;58(10):743–749.
[7] Lykkesfeldt J, Poulsen HE. Is vitamin C supplementation beneficial? Lessons learned from randomised controlled trials. British Journal of Nutrition. 2010;103(9):1251-1259. doi:10.1017/S0007114509993229
[8] Wegmüller R, Tay F, Zeder C, Brnic M, Hurrell RF. Zinc absorption by young adults from supplemental zinc citrate is comparable with that from zinc gluconate and higher than from zinc oxide. J Nutr. 2014 Feb;144(2):132-6. doi: 10.3945/jn.113.181487. Epub 2013 Nov 20. PMID: 24259556; PMCID: PMC3901420.
[9] WHO. The World Health Report 2002: reducing risks, promoting healthy lives. Geneva: World Health Organization; 2002.
[10] IZiNCG; Brown KH, Rivera JA, Bhutta Z, Gibson RS, King JC, Lönnerdal B, Ruel MT, Sandtröm B, Wasantwisut E, Hotz C. Assessment of the risk of zinc deficiency in populations and options for its control. Food Nutr Bull. 2004;25:S99–S203
[11] Wegmüller R., Tay F., Zeder C., Brnic M., Hurrell R.F. Zinc absorption by young adults from supplemental zinc citrate is comparable with that from zinc gluconate and higher than from zinc oxide. J. Nutr. 2014;144:132–136. doi: 10.3945/jn.113.181487.