Most people don't consider vitamin A when considering their immune systems, but both vitamin A and vitamin C are needed to guard against infection. Most people, maybe as high as 80 percent, are short in vitamin A. The prevalence of night blindness and the common cold gives convincing evidence of this deficiency.
A study in the Proceedings of the Nutritional Society showed that vitamin A and related retinoids play a major role in immunity, including expression of mucins and keratins, lymphopoiesis, apoptosis, cytokine expression, production of antibody and the function of neutrophils, natural killer cells, monocytes or macrophages, T lymphocytes and B lymphocytes.
Clinical trials suggest that vitamin A supplementation reduces morbidity and mortality in different infectious diseases. But vitamin A is not very stable. Since it is easily destroyed by heat and we cook almost everything, we don't get nearly enough vitamin A. We may consume a generous amount of vitamin A and still have low body levels.
Vitamin A is a fat soluble vitamin and requires normal fat assimilation.
And since the mainstream has advocated a disastrous "low fat" diet for over 30 years, you can imagine the vulnerability.
Carotene (pro-vitamin A), as found in raw carrots, is much more efficient in raising the body's level of vitamin A than fish liver oils. Carotene from carrots can be stored in the liver in quantity, while only a small quantity of true vitamin A can be stored.