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What is the immune system? A complete guide

What is the immune system?

The immune system is an interactive network of organs, white blood cells and proteins that work together to protect the body from outside invaders, such as bacteria, viruses, fungi, or other foreign substances.

When your immune system is working properly, you don’t even notice it. But when you have an under- or over-active immune system, you are at a greater risk of developing infections and other health conditions.

What does the immune system do?

The role of the immune system is to protect the body. It works to prevent disease-causing germs - also known as pathogens - like bacteria, viruses, parasites or fungi from entering the body. If they get in, the immune system kills them or limits their harm. It can also recognise and neutralise harmful substances like toxins and chemicals from the environment, and fight against the body’s own cells that might have changed due to an illness, such as cancer. 

For your body’s natural immune defence to work smoothly, the immune system must be able to differentiate between “self” and “non-self” cells, organisms and substances. 

What are self and non-self substances?

  • Self substances are proteins on the surface of our own cells. Normally, the immune system has already learned at an earlier stage to identify these cell proteins as self, but when it identifies its own body as non-self, and fights it, this is called an autoimmune reaction.

  • Non-self substances are called antigens, which includes the proteins on the surfaces of bacteria, fungi and viruses. Immune cells detect the presence of antigens and work to defend themselves.

Innate immune system

The innate (non-specific) immune system is your body’s first line of immune defence. You are born with this immune system already in place and its role is to prevent harmful germs and other substances from entering your body. 

The innate immune system is a series of barriers that can be separated into two categories: a structural or physical barrier (first line) and a chemical barrier (second line).

The structural components that make up the first line of immune defence against pathogens include:

  • Skin: A waterproof barrier that secretes oil with bacteria-killing properties

  • Lungs: Mucous in the lungs traps foreign particles, and small hairs wave the mucous upwards so it can be coughed out

  • Digestive tract: The mucous lining in the digestive system contains antibodies, and the acid in the stomach can kill most microbes

  • Other defences: Body fluids like sebum (oil from your skin), saliva and tears contain antibacterial enzymes that help reduce the risk of infection. The constant flushing out of the urinary tract and bowel also helps.

The chemical components involved in the innate immune system include a number of immune cells: white blood cells like natural killer cells and phagocytes like macrophages, neutrophils, dendritic cells, mast cells, basophils and eosinophils. 

Innate immunity does not have to be learned through exposure to a microorganism or invader to work effectively. It responds to invaders immediately, without needing to learn to recognise them.  

Adaptive immune system

The innate immune system is often enough to destroy invading microbes. If the innate immune system fails to adequately destroy the pathogen and it manages to penetrate the physical and chemical barriers of the skin and mucous membranes, the third line of defence, the adaptive (specific) immune system, is activated. 

Adaptive immunity is not present at birth and must be learned. The learning process starts when the immune system encounters a foreign invader for the first time and recognises antigens (non-self substances). The white blood cells are the key players in this process. They learn the best way to attack each antigen and begin to develop a memory for that particular antigen so they can mount an immune response quicker and more effectively if they were exposed to it again. They travel through the bloodstream and into tissues, searching for and attacking microorganisms and other foreign invaders such as bacteria, viruses, parasites and fungi.

Organs involved in immune system

In addition to immune cells, the immune system includes several organs dispersed throughout the body. These organs are classified as primary or secondary lymphoid organs.

The primary lymphoid organs are the sites where white blood cells are produced and/or multiply:

  • The bone marrow produces all the different types of white blood cells, including neutrophils, eosinophils, basophils, monocytes, B cells, and the cells that develop into T cells (T cell precursors).

  • In the thymus, T cells multiply and are trained to recognise foreign antigens and ignore the body’s own antigens. T cells are critical for acquired immunity.

When needed to defend the body, the white blood cells are mobilised, mainly from the bone marrow. They then move into the bloodstream and travel to wherever they are needed.

The secondary lymphoid organs include the spleen, lymph nodes, tonsils, appendix, and peyer patches in the small intestine. These organs trap microorganisms and other foreign substances, providing a place for mature cells of the immune system to collect, interact with each other and the foreign substances and generate a specific immune response.

Lymph nodes

The lymph nodes are strategically placed in the body and are connected by an extensive network of lymphatic vessels, called the lymphatic system. The lymphatic system transports microorganisms, other foreign substances, cancer cells, and dead or damaged cells from the tissues to the lymph nodes, where these substances and cells are filtered out and destroyed. Then the filtered lymph is returned to the bloodstream.

Lymph nodes can swell after an infection because acquired immune responses to infections are generated in lymph nodes. Sometimes lymph nodes swell because bacteria that are carried to a lymph node are not killed and cause an infection in the lymph node, known as lymphadenitis.

Compromised immune system 

When the immune system doesn't work properly, it's called an immune system disorder. These include: 

  • Primary immune deficiency: When you're born with a weak immune system

  • Acquired immune deficiency: When you get a disease that weakens your immune system

  • An allergic reaction: When your immune system is too active

  • Autoimmune disease: When your immune system turns against you

Immune system disorders

Underactive immune system & immunodeficiency disorders

An immunodeficiency disorder is where the body cannot generate the right immune responses against invading microorganisms. An underactive immune system does not function correctly and makes people vulnerable to infections. It can be life threatening in severe cases. Some common examples include:

  • Severe combined immunodeficiency (SCID): This is an immune deficiency that is present at birth, where children are missing important white blood cells and are in constant danger of infections from bacteria, viruses, and fungi. 

  • Temporary acquired immune deficiencies: Your immune system can be weakened by certain medicines, such as chemotherapy or other drugs used to treat cancer. It can also happen to people who have had organ transplants and take medicine to prevent organ rejection. Infections like the flu virus, mono (mononucleosis), and measles can also weaken the immune system for a brief time. Additionally, your immune system can be weakened by smoking, alcohol, stress, poor sleep and poor nutrition.

  • AIDS: HIV, which causes AIDS, is an acquired viral infection that destroys important white blood cells and weakens the immune system. People with HIV/AIDS become seriously ill with infections that most people can fight off. These infections are called “opportunistic infections” because they take advantage of weak immune systems. 

Overactive immune system 

If you are born with certain genes, your immune system may react to substances in the environment that are normally harmless. These substances are called allergens. Having an allergic reaction is the most common example of an overactive immune system. Dust, mould, pollen, and foods are common allergens. Overactivity of the immune system can take many forms, including: 

  • Allergies and asthma: Allergies are an immune-mediated inflammatory response to normally harmless environmental substances known as allergens. The body overreacts to an allergen, causing an immune reaction and allergy symptoms such as coughing, sneezing, runny nose or itchy rash. This can result in one or more allergic diseases such as asthma, allergic rhinitis, atopic dermatitis and food allergies.

  • Autoimmune diseases: Autoimmune diseases cause your immune system to attack your own body’s cells and tissues in response to an unknown trigger. The cause of autoimmune diseases is unknown, although it is thought to be a combination of a person’s genes and something in the environment that triggers those genes. Examples of autoimmune diseases include rheumatoid arthritis, lupus, inflammatory bowel disease, multiple sclerosis and type 1 diabetes.

What weakens the immune system?

In addition to immune system disorders mentioned above, additional factors can contribute to a weakened immune system.

  1. Insufficient sleep: Lack of sleep, or not getting enough good quality sleep, can make you more prone to catching viruses or germs. This is mainly because your body releases certain proteins , called cytokines, that help the immune system defend against illness only during sleep. 
  2. Low vitamin D: Vitamin D plays a key role in the immune system. It works to modulate both the innate and adaptive immune responses which are equally important in fighting infections.
  3. Poor diet: A diet low in fruits and vegetables and high in processed and refined carbohydrates and sugar can upset the balance of bacteria in the gut, lower your body’s ability to make healthy white blood cells and lead to low levels, or potential deficiencies, in Zinc, Vitamins A, C and E as well as antioxidants and polyphenols.
  4. Smoking: Many of the chemicals found in cigarette smoke can interfere with the immune system by changing the balance of immune cell production and function, therefore causing it to work less effectively. It can also reduce the levels of antioxidants in the bloodstream, including vitamin C levels.
  5. Alcohol: Over consuming alcohol can suppress your immune system, making you more vulnerable to infections caused by bacteria and viruses. 
  6. Lack of exercise: Physical activity acts as a modulator of the immune system. During and after physical exercise, pro- and anti-inflammatory cytokines are released and circulation increases which helps blood get around your body more efficiently so germ-fighting substances can get to where they need to go. 

What is autoimmune disease?

An autoimmune disease is a condition in which your immune system mistakenly attacks your body. Your body loses the ability to tell the difference between self and non-self: what's you and what's foreign. When this happens, the body makes auto-antibodies that attack normal cells by mistake. At the same time, cells called regulatory T cells fail to do their job of keeping the immune system in line. 

The body parts that are affected depend on the type of autoimmune disease, of which there are more than 80 known types. Some autoimmune diseases target only one organ, for example type 1 diabetes damages the pancreas. Other diseases, like systemic lupus erythematosus (SLE), affect the whole body.

What triggers autoimmune disease?

The cause of autoimmune diseases is unknown, although it is thought to be a combination of a person’s genes and something in the environment, such as diet, infections, and exposure to chemicals, triggering those genes. 

Who is most commonly affected by autoimmune disease?

Autoimmune diseases can affect anyone, but certain people are at greater risk, including:

  • Women of childbearing age: More women than men have autoimmune diseases, which often start during their childbearing years.

  • People with a family history: Certain autoimmune diseases like multiple sclerosis and lupus run in families. Not every family member will necessarily have the same disease, but they inherit a susceptibility to an autoimmune condition.

  • People of certain races or ethnic backgrounds: Some autoimmune diseases are more common or affect certain groups of people more severely. For example, type 1 diabetes is more common in white people whereas lupus is most severe for African-American and Hispanic people.

  • People who are around certain things in the environment: Certain environmental exposures such as chemicals, solvents, and viral and bacterial infections are thought to be linked to many autoimmune diseases.

How to support your immune system

Supporting your immune system doesn’t happen overnight. Strengthening your immune system with healthy diet and lifestyle changes is the single best step you can take toward naturally keeping your immune system working properly. Some key factors include: 

  • Eat a diet high in fruits and vegetables: Vitamin A and C rich foods such as butternut squash, carrots, sweet potato red peppers, citrus fruits, broccoli, kiwi and leafy green vegetables are great sources and can help to improve the health of your immune system by providing anti-inflammatory and antioxidant properties. 

  • Exercise regularly: Incorporating physical activity into your daily and weekly regimen is extremely important to strengthen your immune system.

  • Get adequate sleep: When you aren’t getting enough sleep, your immune system won’t be able to function properly. Aim for 7-8 hours of good quality sleep every night. 

  • Try to minimise stress: Stress can suppress protective immune responses and exacerbate pathological immune responses. Incorporate meditation or mindfulness practices daily.

  • Maintain a healthy weight.

  • If you drink alcohol, drink only in moderation: Consuming too much alcohol can negatively impact gut health, decrease immune function and make you more susceptible to harmful pathogens. 

  • Don't smoke.

  • Take protective measures to avoid infection: Wash your hands frequently, for at least 20 seconds each time, and cook meats thoroughly. 

Supplements for immune system


Probiotics are good live bacteria that help you digest the nutrients you eat, which in turn supports your digestive health and immune system. As chronic inflammation is a driver of many diseases and health conditions, the fact that probiotics also have an anti-inflammatory effect in the gut, where 80 percent of the immune system lies, is key for immune health.

Probiotics have been shown to support the stimulation of the immune system and research suggests that some probiotic organisms may enhance nonspecific cellular immune response, including the activation of macrophages and natural killer (NK) cells, as well as induce different cytokine responses (7).

A 2005 study stated that Lactobacillus and Bifidobacteria strains may have potential for preventing a wide scope of immunity-related diseases due to their anti-inflammatory effect (11).

There are many strains of probiotics, which you can get through supplementation and food sources. The two main species of probiotics are Bifidobacteria and Lactobacillus.

Some of the best probiotic rich foods include: 

  • Tempeh
  • Miso
  • Natto
  • Kombucha
  • Raw cheese
  • Kefir
  • Probiotic yogurt
  • Kimchi
  • Pickles
  • Sauerkraut

    Vitamin C

    Vitamin C improves the health of your immune system by providing anti-inflammatory and antioxidant properties. Studies show that getting enough vitamin C, along with zinc, in your diet may help to shorten the duration and reduce symptoms of illnesses like the common cold and respiratory infections (1). Supplementing with up to 1000mg of vitamin C and up to 30mg of Zinc were shown to be effective. 

    Some of the best vitamin C rich foods include: 

    • Citrus fruits: Orange, Grapefruit, Lemon, lime
    • Black Currant 
    • Red and green peppers  
    • Kiwifruit
    • Strawberries
    • Papaya 
    • Green leafy vegetables; Broccoli, kale, parsley 
    • Pineapple 
    • Mango 


      Zinc is known to play a central role in the immune system, and zinc-deficient people experience increased susceptibility to a variety of pathogens. Zinc’s role in the immune system includes, but is not limited to, supporting the barrier of the skin (the immune system's first line of defence), supporting gene regulation, playing a role in the development and function of the cells mediating nonspecific immunity, such as neutrophils and natural killer cells, and also functioning as an antioxidant (5).

      Research shows that this essential mineral can help with production of immune cells and interfere with processes that cause mucus and bacteria to build within the nasal passages. Therefore, zinc has the ability to exert an antiviral effect by attaching to receptors in nasal epithelial cells and blocking their effects.

      Supplementing with zinc may also help to reduce cold-related symptoms and shorten the duration of the common cold when administered within 24 hours of the onset of common cold symptoms (4).

      Food sources of zinc include: 

      • Oysters
      • Red meat and poultry
      • Beans
      • Nuts
      • Certain types of seafood (such as crab and lobster)
      • Whole grains and fortified breakfast cereals
      • Dairy products

        Vitamin D

        Vitamin D helps to modulate the innate and adaptive immune system and a vitamin D deficiency is associated with increased autoimmunity as well as an increased susceptibility to infection. Research proves that vitamin D works to maintain tolerance and promote protective immunity (2).

        The UK government advises everyone to supplement with Vitamin D throughout the months of October to March, with doses not exceeding 4,000IU daily. The need for supplementation is because our bodies cannot make enough vitamin D from the sun during these months (3). It’s also important to supplement with Vitamin D in the summer months if you spend a lot of time indoors and because sun cream - although essential to protect your skin - can block vitamin D absorption.

        Herbs for immune system


        Echinacea is among the most widely used herbal medicines. Most uses of echinacea are for its immunological properties. A 2003 study showed that echinacea demonstrated significant immunomodulatory activities, showing several benefits, including immunostimulation, especially in the treatment of acute upper respiratory infection (9). It also showed maximal effects on recurrent infections, and has been found to have preventive effects on the common cold (8).


        Elderberry has been shown to have a wide array of health benefits, including supporting the process of getting over colds and flus, allergies and inflammation. It is high in vitamin C and other immune-boosting antioxidants. For example, a 2004 study showed that when elderberry was used within the first 48 hours of onset of symptoms, the extract reduced the duration of the flu, with symptoms being relieved on an average of four days earlier (10).


        Ginseng can help support your immune system and fight infections. It has been shown to improve the performance of your immune system by regulating different types of immune cells, including macrophages, natural killer cells, dendritic cells, T cells and B cells. It has also proven to have antimicrobial compounds that work to defend against bacterial and viral infections (12).

        Another study suggested that ginseng extract induces antigen-specific antibody responses when taken orally. Antibodies bind to antigens, such as toxins or viruses, and keep them from contacting and harming normal cells in the body. Because of ginseng’s ability to play a role in antibody production, it helps the body to fight invading microorganisms or pathogenic antigens (13).


        Your immune system is a complex interactive system which works together to protect the body from viruses, bacteria and any foreign substances that can make you unwell. When it is working properly, you don’t even notice it. But when the performance of the immune system is compromised, that’s when you are likely to face illness. 

        Your best chance at keeping your immune system healthy and preventing illness is to eat a whole foods, nutrient-dense diet, get plenty of good quality sleep, exercise regularly, stop smoking, limit your alcohol consumption, and reduce stress. Supplementing with immune supporting vitamins and herbs can also help to keep your body protected and healthy.

        Following these guidelines all year round, and not just in the lead up to the winter months, will give your body the best chance it has to fight off any illnesses and keep your immune system robust and healthy.