factors affecting immune system function in adults

Image Credit: Maggini, Silvia et al. Nutrients vol. 10,10 1531.

Nutrients Important for Resisting and Fighting Infection

By Tracey Walton and Oto Peter Ode BMLS

Having a healthy diet is an important requirement for a robust immune system. Existing healthy eating guidelines include enough of all the nutrients we need.

However, we don't all follow the guidelines. Hectic lifestyles, restricted diets, and changes in our eating habits can all be problematic.

This guide will help you ensure that your diet contains enough of the nutrients that scientific evidence has proven to be particularly important for resisting and fighting infections.

Note: supplements may be beneficial for people with more severe deficiencies in certain situations. However, their effect can be highly dependent on type, dosage, gender, age, DNA  and individual circumstances so it’s best to discuss their use with a medical professional. Micronutrient supplements can be harmful if taken to excess!

In this guide we focus on food sources of nutrients and how we can incorporate them into our daily dietary intake.

A Brief Overview of How the Immune System Works

The immune system protects the body against infection as well as other external and internal threats using three distinct mechanisms, depending on the nature of the threat: (1)

  1. Physical Barriers (e.g., skin, epithelial lining of the gastrointestinal and respiratory tracts)
  2. Biochemical Barriers (e.g., secretions, mucus, and gastric acid)
  3. Immune Cells of different types including antibodies (e.g., Phagocytes, Lymphocytes and IgM)

Three Types of Immunity in Humans

  • Innate immunity: Every individual was born with some level of immunity to “non-self-elements” AKA Invaders. These protective features act as first line defence against invading organisms such as bacteria, viruses, fungi and other microbes that can cause disease(s). This response is usually generic in nature. The mucous membrane of the throat and gut are classical examples of innate immunity..
  • Adaptive (acquired) Immunity: This is a more specific line of defence against invading pathogens. Adaptive immunity develops from exposure to diseases or from immunization through vaccine shots hence the term “acquired”. It is sometimes referred to as immunological memory due to the ability our immune system to recognize previous invaders.
  • Passive Immunity: Another name for passive immunity is “borrowed immunity”. A classical example can be seen in a newborn baby (neonate) who receives antibodies from the mother through the placenta prior to birth, and from breast milk following birth. Passive immunity is short-lived as it only protects the neonate from invaders during the early stages of his or her life.

It is important to note that these systems also protect against native cells that may pose a threat to our well-being. Such cells may be malignant or benign.

Outline of the Innate and Adaptive Immune Systems

Outline of the Innate and Adaptive Immune Systems
Image Credit: Maggini, Silvia et al. Nutrients vol. 10,10 1531.

* The innate immune system comprises anatomical and biochemical barriers, having an unspecific cellular response that targets pathogens before they can trigger an active infection.

** The adaptive immune system involves a specific response that is activated by exposure to pathogens; this works with the innate immune system to reduce the severity and progressiveness of infection.

Definition of Terms

Pathogens:Microorganisms capable of causing diseases.

Antigens: Part(s) of a pathogen that alerts the body of an infection.

Immune cells: These are highly specialised cells that recognise antigens, target, and remove them from the body thereby stopping or preventing an illness.

Significant Micronutrients in Immune Function

It's worth mentioning that all micronutrients are important, and everyone should aim to meet daily recommended amounts. The nutrients covered below are those proven by research to be instrumental in the immune system’s ability to fight and overcome infection.

Vitamin C

Key Roles in Resisting and Fighting Infection

  • Effective antioxidant that protects against free radicals produced when pathogens are destroyed by immune cells
  • Regenerates other important antioxidants to their active state
  • Promotes collagen synthesis, thereby supporting the integrity of epithelial barriers
  • Has roles in antimicrobial and killer cell activities thereby increasing serum levels of antibodies.


How Much Do You Need?

Vitamin C Requirements for Adults
Vitamin C UK Gov DR* USA RDA**
Adult Men 40mg/day 90mg/day
Adult Women 40mg/day 90mg/day
Smokers   +35mg/day
*UK Government Dietary Recommendations, Public Health England(3)
**US Recommended Dietary Allowances, National Institutes of Health(4)

Good Food Sources of Vitamin C

Many fruits and vegetables have good amounts of vitamin C, including potatoes. Vitamin C can’t be stored in the body, so it’s important to eat foods containing the vitamin every day.

Food Vitamin C Content
Half a Red Pepper 101mg
1 medium Orange 88mg
Spring greens, boiled (80g) 61.6mg
Orange Juice, chilled (150ml) 60mg
Orange Juice, UHT (150ml) 46.5mg
Cranberry Fruit Juice Drink (150ml) 45mg
6 Strawberries, raw 41mg
Broccoli, Boiled (80g) 35mg
1 Medium Peach 30mg
Green Salad, (80g) 28mg
Cauliflower, boiled (80g) 24mg
Baby Spinach, raw (80g) 23mg
Spring Onions, raw (50g) 13mg
1 Medium Jacket Potato (200g) 12mg

Impact of Vitamin C Deficiency on Immune responses and the Risk of Infection


  • Increased oxidative damage
  • Increased incidence and severity of pneumonia and other infections
  • Increased susceptibility to infections such as pneumonia Impaired wound healing

Note: Vitamin C deficiency or inadequacy is rare in developed countries. Smokers are more likely to have inadequate levels, but not deficiency.

Vitamin D

Key Roles in Resisting and Fighting Infection

  • Broadly impacts functions of immune cells in both the innate and adaptive immune system, as well as the antigen-presenting cells that links the two arms of immunity.(2)
  • Stimulates immune cell proliferation(1)
  • By stimulating innate antimicrobial immune responses, enhances elimination of invading bacteria, and viruses(2)

How Much Do You Need?

The main source of vitamin D for most people is sunlight – it’s often called the sunshine vitamin.

According to the National Health Service (NHS), during the spring and summer months most people should be able to get all the vitamin D they need from sunlight on their skin.(5)

Vitamin D Requirements for Adults
Vitamin D UK Gov DR* USA RDA**
Adult Men 10mcg/day 15mcg/day
Adult Women 10mcg/day 15mcg/day
Adults Over 70 10mcg/day 20mcg/day
*UK Government Dietary Recommendations, Public Health England(3)
**US Recommended Dietary Allowances, National Institutes of Health(6)

Good food sources of vitamin D

Dietary sources of vitamin D are limited to just a few foods, including oily fish like mackerel, salmon and tuna, egg yolks, liver, fortified breakfast cereals, margarine and other fortified fat spreads.

There are also a number of dairy and dairy alternative products that are fortified, including some yoghurts and vegan milk alternatives. The best way to identify these is to look at labelling and ingredients.

Food Vitamin D Content
Salmon, pink, canned (60g) 8.2mcg
Mackerel Fillet, grilled (90g) 7.6mcg
Smoked Salmon (60g) 5.34mcg
Sardines, canned in tomato sauce (120g) 4mcg
Tuna, fresh, baked (120g) 3.7mcg
1 Medium Boiled Egg 1.8mcg
Soya Milk (200ml) 1.6mcg
Cornflakes (30g) 1.4mcg
2 Pork Sausages 1mcg
Soya Yoghurt (125g) 1mcg
Tuna, canned, drained (60g) 0.66mcg
Low Fat Spread, polyunsaturated (10g) 0.58mcg

The NHS advises supplementation during the autumn and winter months, and all-year-round for people who get very little sunlight.

Impact of Vitamin D Deficiency on Immune Responses and the Risk of Infection

  • Increased susceptibility to infections, especially respiratory tract infections (RTI)
  • Increased morbidity and mortality, increased severity of infections
  • Increased risk of autoimmune diseases


Vitamin D deficiency is more likely in certain groups of people as a result of insufficient sunlight exposure, vulnerability to malabsorption, obesity or ageing.(6)

Vitamin A

Key Roles in Resisting and Fighting Infection

  • Helps maintain structural and functional integrity of mucosal cells in innate barriers (e.g., skin, respiratory tract, etc.)
  • Important for normal functioning of innate immune cells
  • Necessary for generation of antibody responses to antigens
  • Supports anti-inflammatory response


How Much Do You Need?

Vitamin A Requirements for Adults
Vitamin A UK Gov DR* USA RDA**
Adult Men 700mcg/day 900mcg/day
Adult Women 600mcg/day 700mcg/day
*UK Government Dietary Recommendations, Public Health England (3)
**US Recommended Dietary Allowances, National Institutes of Health (7)

Consuming too much vitamin A can have serious consequences, especially if you are pregnant, see NHS information about Vitamin A

Good food sources of vitamin A

There are two forms of vitamin A available to the body – retinol and beta-carotene, the latter of which is converted into vitamin A in the body.

Retinol is found in foods of animal origin such as milk, cheese, butter, egg yolk, liver and oily fish. Margarines are also fortified with vitamin A by law.

Beta-carotene is found mainly in dark green vegetables such as spinach and curly kale, along with yellow, orange and red fruits.

Food Vitamin A Content (mcg)
60g boiled carrots 756
1/2 mango 225
1 boiled egg 116
1 tomato 91
300ml semi-skimmed milk 68
30g reduced-fat Cheddar cheese 54

Impact of vitamin A deficiency on immune responses and the risk of infection

  • Affects the number and activity of many cells involved in immune functions
  • Increased susceptibility to infections including respiratory tract infections


Vitamin E

Key Roles in Resisting and Fighting Infection

  • Protects the integrity of cell membranes from damage caused by free radicals(1)
  • Enhances activity of certain immune cells(1)
  • Has probable protective effects on influenza(2)
  • Reduction in viral load and symptoms after influenza infection(2)

How Much Do You Need?

Vitamin E Requirements for Adults
Vitamin E UK Gov DR* USA RDA**
Adult Men n/a 15mg/day
Adult Women n/a 15mg/day
*Vitamin E is not included in the UK Government Dietary Recommendations – it is found in a wide range of foods and deficiency is rare.
**US Recommended Dietary Allowances, National Institutes of Health 8

Good food sources of vitamin E

Vitamin E tends to be found in foods that are rich in fat such as vegetable oils, margarine, avocado, nuts and seeds.

However, green leafy vegetables, eggs and wholegrains also contain some.

Food Vitamin E Content (mg)
50g sunflower seeds 18.9
50g almonds 12
1/2 avocado 2.4
90g spinach 1.5
1tsp sunflower oil 1.5
1tsp low-fat spread 0.3

Impact of vitamin E deficiency on immune responses and the risk of infection

  • Early studies using animal models have established a clear link between vitamin E deficiency and impairment in immune functions(2)
  • Impairs aspects of adaptive immunity(1)


Key Roles in Resisting and Fighting Infection

Zinc is a nutrient crucial for maintaining both innate and adaptive immunity(2)

Its antioxidant effects protect against free radicals(1)

Plays a central role in cellular growth and differentiation of immune cells(1)

Helps to maintain skin and mucosal membrane integrity(1)

Essential for intracellular development and activation(1)

How Much Do You Need?

Zinc Requirements for Adults
Zinc UK Gov DR* USA RDA**
Adult Men 9.5mg/day 11mg/day
Adult Women 7mg/day 8mg/day
*UK Government Dietary Recommendations, Public Health England (3)
**US Recommended Dietary Allowances, National Institutes of Health (9)

Good food sources of Zinc

Zinc is present in trace amounts in many foods. Good sources include red meat, dairy products, eggs, shellfish, nuts and seeds and wholegrain cereals.

Food Zinc Content
Oysters, 4 medium (200g inc shells) 16.6mg
Braising steak, braised, lean (100g) 9.5mg
Lamb Liver (150g) 8.8mg
Quorn pieces (100g) 7mg
Quarter Pound Beef Burger 6.8mg
Crab, white meat (50g) 3.6mg
Pumpkin seeds (25g) 1.7mg
Pine nuts (25g) 1.6mg
Cashew nuts, roasted and salted (25g) 1.4mg
Tahini Paste (25g) 1.4mg
Sunflower seeds, toasted (25g) 1.3mg
Cheddar Cheese (30g) 1.2mg
Curry powder (25g) 1mg
Cocoa powder (10g) 0.7mg

Impact of Zinc Deficiency on Immune Responses and the Risk of Infection

  • Deficiency negatively impacts immune cell development and functions in both innate and adaptive immunity(2)
  • Increased bacterial, viral and fungal infections (particularly diarrhea and pneumonia)(1)
  • Diarrheal and respiratory morbidity(1)

Note: Inadequate intake of zinc is also present in developed countries, particularly amongst the elderly and those who drink large amounts of alcohol.

Eating Habits and Health

As we said at the beginning of this guide: by following healthy eating guidelines, we can be sure we are getting enough nutrient requirements for healthy living. If you find your eating habits fall short of the guidelines (lots of us struggle to get 5-a-day fruit and veg for example), now could be a good time to try and improve.

Of course, making changes in any area of our lives is usually a daunting task. Our advice would be to take things easy (one day at a time) and strive to be consistent.

Set dietary targets with friends and family. Make sure these goals/targets are achievable in the long term.

By changing our eating habits for the better, we can lead a healthier life and do the things we love with happiness and ease.

Here's some more resources to help you on this journey:

Simple Steps to Healthy Eating

The Eatwell Plate

How Healthy is Your Diet?

You can keep track of how your healthy eating plan is going with the Weight Loss Resources online food diary. Try it free

Take our FREE trial »


  1. Maggini, S., Pierre, A., & Calder, P. C. (2018). Immune Function and Micronutrient Requirements Change over the Life Course. Nutrients, 10(10), 1531. doi:10.3390/nu10101531
  2. Wu, D., Lewis, E. D., Pae, M., & Meydani, S. N. (2019). Nutritional Modulation of Immune Function: Analysis of Evidence, Mechanisms, and Clinical Relevance. Frontiers in immunology, 9, 3160. doi:10.3389/fimmu.2018.03160
  3. Public Health England Government Dietary Recommendations
  4. National Institutes of Health Vitamin C Health Professional Fact Sheet
  5. NHS Vitamin D
  6. National Institutes of Health Vitamin D Health Professional Fact Sheet
  7. National Institutes of Health Vitamin A Health Professional Fact Sheet
  8. National Institutes of Health Vitamin E Health Professional Fact Sheet
  9. National Institutes of Health Zinc Health Professional Fact Sheet
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