Microorganisms (including pathogen microorganisms) invade our bodies from birth with an intention to replicate in a moist, nutrient-rich, and adequate temperate environment. Although, the invasion of these microorganisms could result in infection and illness, not all of them are harmful.
A typical beneficial microorganic microbiota is found in the gut. They maintain a symbiotic relation with its host, which can be beneficial in improving the host’s gastrointestinal tract. The microbiota competes with pathogen microorganism to some extent to protect the host. However, some of the pathogenic microorganisms use highly specialised mechanisms to survive and multiply hence enabling them to invade the host’s body. The inhabit various nutritional compatible niches within the host’s body where they replicate and possibly spread to a new host. At this point the body depends on the immune system to defend it from the new invader.
The immune system is an organised cells and molecules with specialised roles to defend the host’s body against infections. There are two different types of responses by the immune system.
1. Innate/natural responses – these are non-specific and acts fast mostly within minutes/hours. The first line of these immune responses are physical barriers such as the skin, body, hair, and mucus membrane. If these barriers are breached then so called “myriad” immune cells (i.e., leukocytes such as natural killers (NK) cells, neutrophils and macrophages) while cytokines repair the damages caused by the invaders  .
2. Adaptive/acquired immune system – these are specific and act slower than the innate responses. Once a specific pathogens and foreign tissues activates the adaptive immune cells such as the T and B cells, which recognise specific invading microorganism and recruit antibodies against it. These antibodies either mark the invading foreign tissue or pathogen for attack by other immune cells or counteract the pathogens themselves  .
The processes involved in each step of the immune response depends on the presence of specific micronutrients to function effectively. The first demonstration/identification of the importance of micronutrients was in 1753 when James Lind observed the recovery of a group of men that consumed citrus fruit and made a remarkable recovery to scurvy caused by vitamin C deficiency .
In this article, we will review important micronutrients that are essential to the normal functioning of the immune system. We will also identify the effects of inadequate consumption of these micronutrients on the risk of infection. It is hence important to use supplements as a source of these micronutrients to keep the immune system healthy throughout the year as an effective approach to preventing infection and disease.
Here are 10 important supplements that are known to potentially boost the immune system.
1. Vitamin A (retinol)Vitamin A is known for its significant role in enhancing the immune function due to its anti-inflammation properties. It also plays a crucial role in maintaining vision, promoting growth and development, and protecting the epithelium and mucus integrity of the body.
Vitamin A has been shown to demonstrate a therapeutic effect in the treatment of infection diseases. It plays an important in maintain the physical barriers including the mucous barriers in the eyes, lungs, gut, and genitals which help trap bacteria and other infectious agents .
Vitamin A helps to provide antimicrobial effects, production and development of antibody and responses to antigen. They also play a role in anti-inflammation, antioxidant effects and oxidative burst effects.
Deficiencies in vitamin A has been shown to lead to increased susceptibility in infections and slower recovery time to illness.
2. Vitamin DVitamin D is obtained from the diet or mostly synthesised in the skin after exposure to UV B-light.
Vitamin D has been used in the past to treat infections like tuberculosis. Patients were previous required to visit a sanatorium where they are exposed to sunlight and was believed to kill the microorganism causing the infection.
Vitamin D is known to play a significant role in enhancing the antigen fighting effect of monocytes and microphages. The receptor of vitamin D is expressed on the immune cells including B cells, T cells and antigen presenting cells. These cells can synthesise active vitamin D metabolites .
There have been increased amount of epidemiological evidence that suggests that the deficiency of vitamin D can be linked to autoimmune diseases such as rheumatoid arthritis, diabetes mellitus, multiple sclerosis, and inflammatory bowel .
Vitamin D is a crucial vitamin in promoting calcium homeostasis which promotes bone health. It enhances the absorption of calcium in the small intestine and incites osteoclast differentiation and calcium reabsorption of bone.
Vitamin D deficiency can lead to increased susceptibility to infection and autoimmunity especially to a genetically susceptible host.
3. Vitamin CVitamin C is a well-known supplement that can be used to improve the immune system. It can be used to protect against infections due to its function in immune health.
Vitamin C supports collagen synthesis and protects cell membrane from damage caused by free radicals which means they are important in supporting the integrity of the epithelial barriers .
It is a powerful antioxidant that neutralise free radicals hence protecting the cells against damages induced by oxidative stress hence protecting leukocytes and lymphocytes from oxidative stress . High dose of vitamin C can stimulate the activities of phagocytic and T-lymphocytic activities.
High doses can help severely ill individuals recover rapidly . Research has demonstrated that supplementing with vitamin C can reduce the extent and severity of common cold as well as other upper respiratory tract infections .
Hemila and Chalker 2013, conducted a study on 11,306 participants in 29 trials and concluded that regularly supplementation at an average dose of 1000mg to 2000mg per day of vitamin C reduced the extent of colds in adults and children by 8% and 14% respectively 
Deficiency of vitamin C can lead to increased oxidative damage , decreased DTH response  and weakened wound healing . Severe deficiency of vitamin C can result in development of scurvy. This is characterised by the diminishing of collagenous structures leading to a poor wound healing and weakened immunity . Furthermore, individuals with vitamin C deficiency will experience increase in incidence and severity of pneumonia and other infections  .
4. ZincZinc is a vital micronutrient in multiple components of biological processes. It is one of the most abundant trace metals in organisms as it is important in various cell processes .
Zinc helps maintain the function of the immune system, helps in wound healing, and improve normal growth. Supplementation of Zinc is beneficial to the intestinal immune functions, increases cytotoxicity of NK cells, restores thymulin activities, increases the numbers of cytotoxic T-cells and reduces the number of activated T helper cells which contributes to autoimmunity .
Severe zinc deficiency can result in skin lesions and impaired wound healing, growth retardation, mental retardation, hypogonadism, anaemia, diarrhea, anorexia, and impaired visual and immunological function .
Decreased resistance to infection caused by zinc deficiency can be more prevalent and severe in older people and children .
5. Grapeseed ExtractGrapeseed extract (Vitis vinifera) is the best-known source of proanthocyanins. They are naturally occurring compounds in fruits, vegetables, seeds, flowers, bark, and nuts.
The proanthocyanins in grape seeds have shown to have some biological effects including inhibition of oxidative stress, protection of immune system, inflammation, and prevention of photo-carcinogenesis (caused by overexposure of skin to ultraviolet radiation resulting in skin cancer). In vitro and in vivo study demonstrate a protective effect of grape seed proanthocyanins against photo-carcinogenesis .
It also contains high antioxidants which includes anthocyanins, flavonoids, oligomeric proanthocyanin complexes and phenolic acids .
Grape seed extract has shown to prevent the growth common foodborne bacteria that are responsible for food poisoning and abdominal distress such as Campylobacter and E. coli. It is also widely used as a traditional remedy for candida (a common yeast-like fungus that can result in thrush)  . Experimental studies have also demonstrated the inhibition of 43 strains of antibiotic-resistant Staphylococcus aureus bacteria .
Studies has shown that grape seed extract can enhance wound healing which has been linked to high levels of proanthocyanins in grape seed extract .
6. IronIron is a vital micronutrient in multiple components of biological processes. It is one of the most abundant trace metals in organisms as it is important in various cell processes. It can shift between various oxidation states which makes it an essential co-factor in electron transfer and oxidation-reduction reactions, and allows it to interact reversibly with other atoms, especially oxygen, sulphur, and nitrogen .
Iron is an essential for immune system and is very involved in the differentiation and growth of epithelial tissue hence maintain the structure and functional integrity of mucosal cells in innate barriers. It also has antimicrobial, inflammation, and antioxidant effects as it is involved in the regulation of cytokine production and action .
Iron supplements can be beneficial for people who find it challenging to take in adequate iron from dietary measures alone, such as in a plant-based diet. A shortage of iron in the blood can lead to a range of serious health problems, including iron deficiency anaemia.
Iron deficiency and excess can both cause adverse effects on cells as free iron can be cytotoxic when present at high concentrations by catalysing the development of oxidative radicals that damage proteins, lipids, and nucleic acids .
7. ElderberryElderberry have historically been used both as food and a remedy to prevent and treat illness . It contains a subset of flavonoids known as anthocyanins with possibly possess an immunomodulation and anti-inflammatory properties . Anthocyanins are shown to possess antiviral properties. They do so by binding to glycoproteins on viral cells responsible for enabling viruses to enter the host cells, hence inhibiting viral infections .
Elderberry has demonstrated inhibitory effect on cold and influenza. It has been shown to be effective in the treatment of upper respiratory symptoms and can be helpful in reducing the extent of colds and influenza . A few studies have demonstrated the in vitro inhibitory effect of elderberry on influenza A and influenza B viruses  .
Elderberry may also influence the immune system cytokines in addition to its direct effect on viruses. Recent research finding suggest that elderberry could potentially be used as a nutraceutical to modulate stress and immune response in vertebrates .
8. Black GarlicBlack garlic is achieved from the fermentation of fresh garlic (Allium sativum L.) at a controlled high temperature and humidity over time, without adding other treatments or materials. This process decreases the usual strong aroma of fresh garlic by decreasing allicin content. A new chemical content and formulations are formed through this process .
Black garlic possess antioxidant, antibacterial and immunomodulation properties which can be beneficial in protecting and maintaining the human body .
The antioxidants of black garlic are significantly greater than fresh garlic. This is because the fermentation process converts the allicin into antioxidants like alkaloids and flavonoids . These compounds protect the cells from oxidative stress which could potentially result in various diseases.
Compounds in Aged black garlic has been shown to modulate cells associated to the immune system such as cytokines and could potentially serve as an anti-inflammatory agent .
9. SeleniumSelenium is a vital micronutrient in multiple components of biological processes. It plays a major role metabolism and thyroid function. It can act as an antioxidant in extracellular space i.e., the cell cytosol which connects with the cell membranes specifically in the gastrointestinal tract .
Selenium is essential in immune system. Its antioxidant properties help decrease oxidative stress in cells which reduces inflammation and improves immunity. Supplementation of selenium may help boost the immune system of people with influenza, hepatitis C and tuberculosis .
Deficiency of selenium could potentially affect the immune cells which may result in poor immune response . Studies has shown that people with HIV may be susceptible to a risk of death and disease progression when associated to selenium deficiency .
10. B complex vitaminsVitamin Bs including Vitamin B6 (pyridoxine), Vitamin B9 (folate), and Vitamin B12 (cobalamin) are important vitamins in the body immune system.
Pyridoxine is vital in strengthening the immune system. Animal and human studies suggest that vitamin B6 deficiency affects both humoral and cell-mediated immune responses  .
Folate is vital in regulating immune response in the body. It also plays a major role specifically in the survival of regulatory T cells and further preservation of immunological homeostasis in the intestine .
Cobalamin play a crucial role in the healthy balancing of the immune system . They also play a major role in the formation of red blood cells and a co-enzyme in the methylation reactions that occur in health body processes . Its deficiency could result in haematological, neurological, and cognitive disorders such as memory loss, visual disorder, and megaloblastic anaemia .
Deficiency of Vitamin B complex may cause health issues such poor immune response and anaemia resulting in production of abnormal large red blood cells with impaired functionality  .
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