There are three vitamins that secure the proper functioning of one's immune system, and those are vitamin A, C, and D!
a) Vitamin A: is a very important fat-soluble vitamin. It supports the integrity of the mucus membranes. Mucus membranes cover all the internal lining, including the sinuses, throat, esophagus, stomach, all of the intestine, lungs, so, a huge surface area of mucus membranes. 400 square meters, the size of two tennis courts, or one large professional basketball court.
It acts as a barrier to prevent pathogens from getting inside the body. We can see it as the "wall". The first layer (of mucus) has anti-microbial enzymes. It serves like a "flytrap". It traps pathogens. It is filled with T-regulatory cells (let's remember, one of the purposes of that cell is to differentiate good guys from bad guys. T-regulatory cells are able to tag the bad guys). Some antibodies are also involved in the process of pathogen-neutralization.
There is approximately one to one and a half-liter of mucus that is produced every single day.
One of the main guards that protect a wall is the macrophage. That cell eats, and encapsulates pathogens, then dissolves them with chemicals for which production we need sodium (sea-salt). So, it is important to have sodium chloride in the form of sea salt (on a regular basis).
Vitamin A has a profound effect on macrophages. It also has a very important function in keeping the tight junctions in your gut tight, so they don't leak.
A leaky gut may indicate vitamin A deficiency!
So, the "wall" is made out of cells that must be very very tight, for which we need vitamin A. If we are low in vitamin A, pathogens can invade.
One of the best sources of vitamin A is cod liver oil. It has a nice balance of vitamin A, and vitamin D. Some other great sources of vitamin A are butter, egg yolks, fish, and beef liver. Some vegetables also have vitamin A, but in the precursor forms, so they have to be converted to the active form of vitamin A, which is retinol.
b) Vitamin C
Most people know vitamin C is good for colds, flu, etc., but they don't know why it works. The thing you need to know is that your white blood cells should be "saturated" with vitamin C, because they need it in order to perform their defense mechanisms, in various ways.
In fact, white blood cell is 50-100x more concentrated in vitamin C than in your blood, so, when you are deficient in vitamin C, then the white blood cells cannot move around fast, means the vitamin C increases the motion of the white blood cells.
Vitamin C also increases phagocytosis. In other words, they allow the phagocytes to eat up the viruses and bacteria. Their weaponry is eating microbes. When white blood cells have enough vitamin C, there is more "microcytotoxicity". In other words, they have the ability to be very toxic to pathogens.
Vitamin C allows the macrophages to basically eat up all the dead neutrophils and keep things clean. It also enhances the growth and the reproduction of T-cells, and B-cells. Having enough vitamin C will decrease the duration of an infection.
However, make sure that vitamin C you take is food-based because, in nature, it always comes in a complete complex of many things. It doesn't just come in one form called "ascorbic acid".
90 mg daily is a recommended intake value, but if you are trying to prevent infection, then you should be taking 200 mg (daily). If you're going through an infection, take 500 mg daily. By doing that you'll make sure the white blood cells are saturated with enough vitamin C to do their job.
IMPORTANT: if you're eating sugar, that competes for vitamin C. Another thing that can "suck out" a lot of vitamin C is inflammation (or infection).
By not having enough vitamin C you impair your immune system and increase the amount of inflammation (which then depletes your vitamin C).
Having enough vitamin C can reduce the number of colds, viral infections, usually by half (if you are physically active).
Suggestion: if you have a history of weakness within the lungs, make sure you take a little more vitamin C on a regular basis.
c) Vitamin D: is intimately connected to every single immune reaction. White blood cells have the ability to make vitamin D active. One of the viruses' strategies is to turn off or downgrade vitamin D receptors (so you can't get any more vitamin D).
Vitamin D deficiencies occur when there is not enough sun (in the winter, for example). That explains the flu spikes and other viral infections...
Vitamin D is a natural anti-inflammatory steroid. When you have an autoimmune disease, the thing that really creates damage is the inflammatory process.
There are two compounds our immune system makes as antimicrobials, and both need vitamin D. The first one is called cathelicidins, and it is produced by phagocytes (macrophages and neutrophils to act as a weapon to kill off bad microbes).
Cathelicidins damage the wall of the cell, the cellular membrane of the pathogen, and punch a hole in it, if there is not enough vitamin D, you'll have too much inflammation. One side effect would be psoriasis.
The next chemical is defensin, for which production vitamin D is essential. Defensin is needed in the mucous membrane! It is very anti-microbial. If you lack vitamin D, this can get out of control and create rosacea.
It is virtually impossible to get vitamin D from food. Ideally, you get It from the sun, however, Cod liver oil is an excellent source of vitamin D, vitamin A, Omega-3, DHA, and EPA.