Despite thousands of peer-reviewed reviews and studies showing otherwise, many continue to treat food as a simple mix of macro and micronutrients, a composite of fats, proteins, carbs, vitamins and minerals that should be viewed primarily through the lens of portion control. This neglects the incredible biochemistry of our bodies. And it ignores the vital role of the gut microbiome, as well as the effects of phytonutrients like quercetin and rutin on our health.

What Is Our Immune System?

On a daily basis, we are constantly exposed to potentially harmful microbes of all sorts. Our immune system, a network of intricate stages and pathways in the body, protects us against these harmful microbes as well as certain diseases. It recognizes foreign invaders like bacteria, viruses, and parasites and takes immediate action. Humans possess two types of immunity: innate and adaptive.

Innate immunity is a first-line defence from pathogens that try to enter our bodies, achieved through protective barriers. These barriers include:

• Skin that keeps out the majority of pathogens
• Mucus that traps pathogens
• Stomach acid that destroys pathogens
• Enzymes in our sweat and tears that help create anti-bacterial compounds
• Immune system cells that attack all foreign cells entering the body

Adaptive or acquired immunity is a system that learns to recognise a pathogen. It is regulated by cells and organs in our body like the spleen, thymus, bone marrow, and lymph nodes. When a foreign substance enters the body, these cells and organs create antibodies and lead to multiplication of immune cells (including different types of white blood cells) that are specific to that harmful substance and attack and destroy it. Our immune system then adapts by remembering the foreign substance so that if it enters again, these antibodies and cells are even more efficient and quick to destroy it.

Other conditions that trigger an immune response

Antigens are substances that the body labels as foreign and harmful, which triggers immune cell activity. Allergens are one type of antigen and include grass pollen, dust, food components, or pet hair. Antigens can cause a hyper-reactive response in which too many white cells are released. People’s sensitivity to antigens varies widely. For example, an allergy to mould triggers symptoms of wheezing and coughing in a sensitive individual but does not trigger a reaction in other people.

Inflammation is an important, normal step in the body’s innate immune response. When pathogens attack healthy cells and tissue, a type of immune cell called mast cells counterattack and release proteins called histamines, which cause inflammation. Inflammation may generate pain, swelling, and a release of fluids to help flush out the pathogens. The histamines also send signals to discharge even more white blood cells to fight pathogens. However, prolonged inflammation can lead to tissue damage and may overwhelm the immune system.

Autoimmune disorders like lupus, rheumatoid arthritis, or type 1 diabetes are partly hereditary and cause hypersensitivity in which immune cells attack and destroy healthy cells.

Immunodeficiency disorders can depress or completely disable the immune system, and may be genetic or acquired. Acquired forms are more common and include AIDS and cancers like leukaemia and multiple myeloma. In these cases, the body’s defences are so reduced that a person becomes highly susceptible to illness from invading pathogens or antigens.

What factors can depress our immune system?

• Older age: As we age, our internal organs may become less efficient; immune-related organs like the thymus or bone marrow produce less immune cells needed to fight off infections. Ageing is sometimes associated with micronutrient deficiencies, which may worsen a declining immune function.
• Environmental toxins (smoke and other particles contributing to air pollution, excessive alcohol): These substances can impair or suppress the normal activity of immune cells.
• Excess weight: Obesity is associated with low-grade chronic inflammation. Fat tissue produces adipocytokines that can promote inflammatory processes. Research is early, but obesity has also been identified as an independent risk factor for the influenza virus, possibly due to the impaired function of T-cells, a type of white blood cell. [2]
• Poor diet: Malnutrition or a diet lacking in one or more nutrients can impair the production and activity of immune cells and antibodies.
• Chronic diseases: Autoimmune and immunodeficiency disorders attack and potentially disable immune cells.
• Chronic mental stress: Stress releases hormones like cortisol that suppresses inflammation (inflammation is initially needed to activate immune cells) and the action of white blood cells.
• Lack of sleep and rest: Sleep is a time of restoration for the body, during which a type of cytokine is released that fights infection; too little sleep lowers the amount of these cytokines and other immune cells. 

Does an Immune-Boosting Diet Exist?

Eating enough nutrients as part of a varied diet is required for the health and function of all cells, including immune cells. Certain dietary patterns may better prepare the body for microbial attacks and excess inflammation, but it is unlikely that individual foods offer special protection. Each stage of the body’s immune response relies on the presence of many micronutrients. Examples of nutrients that have been identified as critical for the growth and function of immune cells include vitamin C, vitamin D, zinc, selenium, iron, and protein. They are found in a variety of plant and animal foods.

Diets that are limited in variety and lower in nutrients, such as consisting primarily of processed foods and lacking in minimally processed foods, can negatively affect a healthy immune system. It is also believed that a Western diet high in refined sugar and red meat and low in fruits and vegetables can promote disturbances in healthy intestinal microorganisms, resulting in chronic inflammation of the gut, and associated suppressed immunity. 

The microbiome is an internal metropolis of trillions of microorganisms or microbes that live in our bodies, mostly in the intestines. It is an area of intense and active research, as scientists are finding that the microbiome plays a key role in immune function. The gut is a major site of immune activity and the production of antimicrobial proteins. The diet plays a large role in determining what kinds of microbes live in our intestines. A high-fibre plant-rich diet with plenty of fruits, vegetables, whole grains, and legumes appear to support the growth and maintenance of beneficial microbes. Certain helpful microbes break down fibres into short-chain fatty acids, which have been shown to stimulate immune cell activity. These fibres are sometimes called prebiotics because they feed microbes. Therefore, a diet containing probiotic and prebiotic foods may be beneficial. Probiotic foods contain live helpful bacteria, and prebiotic foods contain fibre and oligosaccharides that feed and maintain healthy colonies of those bacteria.

• Probiotic foods include kefir, yoghurt with live active cultures, fermented vegetables, sauerkraut, tempeh, kombucha tea, kimchi, and miso.
• Prebiotic foods include garlic, onions, leeks, asparagus, Jerusalem artichokes, dandelion greens and seaweed. However, a more general rule is to eat a variety of fruits, vegetables, beans and whole grains for dietary prebiotics.