Child and Adult: Comparing Immune Systems

Immunity and Survival

Humans are social creatures, often eating for pleasure in group settings. While humans may love and appreciate the process and the feelings associated with eating food, there is little thought about the immune and survival advantages provided by the macro- and micronutrients inherent to food. Children are no strangers to eating for pleasure; sweetness dominates the taste preferences of most. It is paramount to understand the baseline biological processes that promote immune health in children versus adults. How do modern food preferences affect a child’s immune biology?

A child’s basic immunology is imperfect up until approximately the age of five years. Until this point, a child’s immune system has not been adequately trained to respond adaptively via antibodies to invading novel pathogens, leaving them at risk for overwhelming infectious disease. History has shown this truth with excessive death rates in the very young and the very old before the advent of vaccinations and sanitation. However, despite this, the best childhood health outcomes are predicated on providing the immune system with the prerequisite nutrients for optimal function, including breastmilk and diverse, whole food-based nourishment.

The Developing Immune System: Tolerance is Key

From the very beginning of life, a child must learn to tolerate the outside environment through specific immune events where antigen presenting cells differentiate between harmless and harmful particles that enter the body: for example, a peanut protein versus a dust mite protein or a pneumococcal bacterium. In the first two years of life, the immune system is repetitively bombarded with foreign protein antigens until it has seen and responded to all in either a pathogenic or tolerant form, a process called immune priming to tolerance. This is the primary result of the evolutionarily beneficial childhood proclivity toward placing all objects in the mouth after six months of age. The primary food nutrient drivers of tolerance are vitamins A and D, zinc, and fiber.1-4

Child vs. Adult Immune System

In comparison, the average, properly trained adult immune system has fully developed tolerance, rendering it prepared to defend against pathogens at all times until advancing age reduces function of T helper cells and antigen presenting cells.

Children develop fully functional innate immune pathogen sensing and killing systems in place by two years of age.₅ The innate immune system is a sensing system that recognizes patterns of protein fragments that appear dangerous and rapidly mounts a local and deadly response. The innate system deploys pattern recognition receptors all over the child’s body to recognize the abnormal pattern of a pathogen and locally kill it with inflammasomes, neutrophil extracellular traps, macrophages, and more. These dead pathogens are then presented to the adaptive immune system via antigen presenting cells like dendritic cells, Kupffer cells, glial cells, and others. This process uses T cells and B cells to ultimately develop long lasting antibodies and memory to this pathogen, making future encounters less problematic.

The process of innate pathogen killing is much more profound in a child versus an adult, as children have naïve adaptive immunobiology before initial exposure to a pathogen. Consuming the precursor macro- and micronutrients for effective innate immune activity is critical for a child’s health. Once a pathogen has been seen and thwarted, the child’s future pathogen-specific immune response mirrors that of the adult through memory B and T cell activity.

Immunological Fade

Pathogen memory is the key attribute of adaptive immunity that prevents succumbing to a pathogen. The systems that protect a child and young adult begin to fade in late adulthood. This immunological fade can occur in the younger cohort for many of the same reasons as the elderly other than cellular senility based purely on age:

• Macronutrient excesses drive inflammatory-based immune dysfunction; insulin resistance with excess pro-inflammatory free fatty acids and glucose/fructose concentration gradients driving innate immune and T helper cell dysfunction over time
• Micronutrient deficiencies slow protein production, immune cell function, and cell signaling pathways critical to immune regulation; vitamins A, B, C, D, and E, zinc, iron, selenium, magnesium, and copper play the greatest roles₄
• Excessive chemical exposures slow cellular protein production, function, and cell signaling pathways critical to immune regulation₆
• Sedentary behaviors drive immune dysregulation at many levels primarily via insulin resistance and metabolic derangements
• Dietary and pharmacologically induced intestinal and pulmonary microbiome damage tilt toward an inflammatory phenotype, further reducing immune function

A modern diet, which includes a large number of processed foods, is a major problem for immune health in both adults and children. Insulin resistance is a primary driver of hyperglycemia, glycation reactions, and hyperlipidemia, and it is mediated by the chronic and excessive consumption of free fatty acids primarily coupled to a sugar gradient of glucose and/or fructose. A classic example would be a fast-food meal: a cheeseburger, French fries, and a 16-ounce soda._7,8 The most important variables appear to be frequency and volume of fatty acids and sugars consumed, as chronic consumption may lead to the development of a positive calorie flux.

Over many years, diet-induced chronic metabolic changes usher in a period of immune dysregulation, notably decreasing effective pathogen killing and increasing metabolic disease risk._9,10 Inflammation actually reinforces the dietary induced insulin resistance metabolic derangement pathway to repeat itself in a continuous cycle.

The counterweight diet to the highly processed modern diet is the healthy Mediterranean diet which is loaded with whole, minimally processed, natural foods that have little ability to cause insulin resistance and metabolic immune derangements.

Micronutrient insufficiencies occur commonly in modern children and adults. As micronutrients are the cofactors for most cellular enzymatic reactions in the body, the consequences of insufficiency are slowed protein production, deranged immune cell function, and disrupted immune cell signaling pathways critical to immune regulation among others. Vitamins A, B, C, D, and E, zinc, iron, selenium, magnesium, and copper play the greatest roles.₁₁ Each of these micronutrients is involved in different and also cooperative events in the immune system. There is a synergy available when micronutrients are consumed as whole foods where iron and vitamin A are found near each other in liver or vitamin E and selenium in nuts. It makes the most evolutionarily and mechanistic sense that humans were meant to maintain immune health via direct synergistic whole food intake.

The greatest difference between the adult and the child’s immune system is the time course by which the above lifestyle behaviors have inflamed and crippled the adult immune system. The ever-present reality that children are now more sedentary, consume more metabolically problematical foods, and are exposed to more chemicals has placed massive negative pressure on their immune health, like an aged adult.

References

1. Tan, J., McKenzie, C., Vuillermin, P. J., Goverse, G., Vinuesa, C. G., Mebius, R. E., Macia, L., & Mackay, C. R. (2016). Dietary Fiber and Bacterial SCFA Enhance Oral Tolerance and Protect against Food Allergy through Diverse Cellular Pathways. Cell reports, 15(12), 2809–2824. https://doi.org/10.1016/j.celrep.2016.05.047
2. Veldhoen, M., & Brucklacher-Waldert, V. (2012). Dietary influences on intestinal immunity. Nature reviews. Immunology, 12(10), 696–708. https://doi.org/10.1038/nri3299
3. Chinthrajah, R. S., Hernandez, J. D., Boyd, S. D., Galli, S. J., & Nadeau, K. C. (2016). Molecular and cellular mechanisms of food allergy and food tolerance. The Journal of allergy and clinical immunology, 137(4), 984–997. https://doi.org/10.1016/j.jaci.2016.02.004
4. Gombart, A. F., Pierre, A., & Maggini, S. (2020). A Review of Micronutrients and the Immune System-Working in Harmony to Reduce the Risk of Infection. Nutrients, 12(1), 236. https://doi.org/10.3390/nu12010236
5. Simon, A. K., Hollander, G. A., & McMichael, A. (2015). Evolution of the immune system in humans from infancy to old age. Proceedings. Biological sciences, 282(1821), 20143085. https://doi.org/10.1098/rspb.2014.3085
6. Mokarizadeh, A., Faryabi, M. R., Rezvanfar, M. A., & Abdollahi, M. (2015). A comprehensive review of pesticides and the immune dysregulation: mechanisms, evidence and consequences. Toxicology mechanisms and methods, 25(4), 258–278. https://doi.org/10.3109/15376516.2015.1020182
7. Nowotny, B., Zahiragic, L., Krog, D., Nowotny, P. J., Herder, C., Carstensen, M., Yoshimura, T., Szendroedi, J., Phielix, E., Schadewaldt, P., Schloot, N. C., Shulman, G. I., & Roden, M. (2013). Mechanisms underlying the onset of oral lipid-induced skeletal muscle insulin resistance in humans. Diabetes, 62(7), 2240–2248. https://doi.org/10.2337/db12-1179
8. Reaven G. M. (1988). Banting lecture 1988. Role of insulin resistance in human disease. Diabetes, 37(12), 1595–1607. https://doi.org/10.2337/diab.37.12.1595
9. Villarreal-Calderón, J. R., Cuéllar, R. X., Ramos-González, M. R., Rubio-Infante, N., Castillo, E. C., Elizondo-Montemayor, L., & García-Rivas, G. (2019). Interplay between the Adaptive Immune System and Insulin Resistance in Weight Loss Induced by Bariatric Surgery. Oxidative medicine and cellular longevity, 2019, 3940739. https://doi.org/10.1155/2019/3940739
10. Patel, P. S., Buras, E. D., & Balasubramanyam, A. (2013). The role of the immune system in obesity and insulin resistance. Journal of obesity, 2013, 616193. https://doi.org/10.1155/2013/616193