Tea and Immunity: What Research Suggests About Their Connection

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Flu or gastro, winter puts a strain on our immune system. Strengthening our line of defense against pathogens is important to avoid falling ill. Conversely, a dysfunctional immune system can be the cause of so-called autoimmune diseases such as rheumatoid arthritis. Controlling our immune system is the key to good health. How can tea help?

Our bodies are subject to numerous daily aggressions: pathogens (bacteria, viruses, etc.), wounds, chronic stress. To defend itself, the body sets up a veritable bunker: the immune system. It's not doliprane or ibuprofen that can get rid of that nasty flu, but our immunity. But how does it work?

The immune system, our body's bastion

Cutting your finger while cooking is a fairly common occurrence, yet it never kills you, despite the thousands of pathogens that enter the wound. In the skin and blood, there are immune cells that can immediately detect the presence of pathogens entering the body. Immune cells recognize specific molecules on the surface of pathogens. Each immune cell has a specific function: some destroy the pathogen, while others absorb it to alert other cells in the skin. Other immune cells secrete molecules called cytokines. These are pro-inflammatory and attract immune cells further away: this is inflammation. Some cytokines dilate blood vessels to facilitate the passage of immune cells, while others activate and attract immune cells directly to the pathogen. When you see that the cut on your finger is red and swollen, with a pulsating sensation of pain, this is inflammation in action.

But let's say you've cut yourself in the jungle and you don't have a first-aid kit. The wound will become infected, and the initial innate immune response is no longer sufficient. At this point, the adaptive immune system kicks in. Immediate response cells that have absorbed pathogens will present them to much more specialized immune cells: lymphocytes. There are two families: B and T.

Both families of lymphocytes will detect molecules on the surface of pathogens, known as antigens. B lymphocytes (LB) secrete molecules called antibodies, capable of binding to a specific antigen. The antibodies produced are detected by the immune system, and the immune cells bind to them, triggering a variety of responses: the so-called specialized, adaptive response. T lymphocytes (LT) are capable of detecting antibodies and antigens. They are divided into several types, some capable of secreting cytokines to modify the activity of the immune system against the pathogen, others to directly destroy pathogens or to prevent overactivity of the immune system.

To avoid having to do all this work again, the immune system stores the information for a certain period of time (several weeks to several years, depending on the frequency of exposure to pathogens): this is the immune memory. If the same pathogen returns, it will be immediately identified and destroyed much more rapidly.

The immune system is complex, depending on both the molecular activities generated by cytokines and the immune cells involved. So how does tea work on the immune system?

Tea and the immune system not so simple ...

While there were many discrepancies in the results, over the past few years a large body of research has come to the same conclusion: tea limits immune activity. In 2013, two American researchers compiled the main findings linking green tea to a reduction in innate and adaptive immune activity. EGCG is the green tea polyphenol of greatest interest in tea research, particularly for its antioxidant effects. In cultured immune cells, in immune cells recovered from the blood of patients with diseases such as cancer, in healthy rats and mice and in inflammatory diseases, EGCG has been shown to reduce the recruitment capacity of innate immune cells and B and T lymphocytes.

Interleukin 8 and interferon gamma are cytokines well known for increasing immune system activity. The majority of these studies have shown that EGCG decreases the secretion of these cytokines, and therefore leads to a reduction in immune system activity. Some studies go even further, showing that immune cells in the blood of breast cancer patients multiply much less in the presence of green tea extracts.

Tea, and green tea in particular, is studied for its immune-regulating properties, which can be a real asset when the immune system goes haywire: autoimmune disease.

When our own body turns against us

Very early in embryonic development, the immune system learns to recognize each part of the organism so as not to destroy it. However, there are times when the immune system fails to do this, triggering innate and adaptive immune responses against our own body: the anti-self response. While there are no fixed explanations as yet, autoimmune diseases may be genetic, but they may also be due to chronic inflammation causing overactivity of the immune system.

One of the most common causes of rheumatism is an autoimmune disease: rheumatoid arthritis. This disease affects the synovial membrane, a fibrous layer that surrounds the ligaments and tendons of the joints. When this is inflamed, immune cells are attracted and destroy the joint cartilage in response to the inflammation. The joints are then progressively destroyed. An Egyptian study on this disease was carried out over 6 months on 120 patients. They consumed 4 to 6 cups of green tea a day and performed specific joint exercises. The results of this clinical study demonstrated an improvement in the disease, in addition to conventional anti-inflammatory treatment. In fact, blood tests showed that additional green tea consumption reduced blood levels of pro-inflammatory molecules such as C-reactive protein. Researchers also noted an improvement in joint condition, with tendons and ligaments appearing less damaged.

But when I'm not ill, will tea attack my immune system?

Although tea limits the activity of the immune system, it would appear that the immune system is not impaired. In fact, some studies have shown that tea stimulates the immune system in specific pathological contexts. In one study, 11 patients drank 6 cups of 100mL Lipton black tea a day for 2 to 4 weeks. Their immune cells were sampled at 2 and 4 weeks. When these cells are exposed to pathogenic molecules, the immune response of the cells is stronger the longer the pathogen/immune cell contact is repeated.

In another study based on immune-compromised rats, the consumption of tea extracts in the form of EGCG, contamination by C.albicans, causing thrush in the mouth, appears to be compromised by an increase in the secretion of immune-activating cytokines such as interleukin 8.

So tea can't help my flu?

In Japan, several human cohorts have demonstrated that drinking green tea infusion or ECGC reduces the risk of contracting the flu virus. More surprisingly, green tea gargles prevent the onset of influenza. While we don't yet know the exact reason for this, it would appear that the tea molecules, particularly EGCG, act directly on the virus rather than boosting the immune system. As the flu virus tends to be contracted through the airways, it's conceivable that green tea might limit the virus's entry, making it easier for the immune system to destroy any remaining viral particles.

The immune system is one of the most difficult biological mechanisms to understand. It depends on a large number of cell types and a wide range of molecules. Some research, notably on influenza and in contexts of immune failure, has yet to be fully explored. However, all the results obtained with green tea, EGCG and pure tea extracts are interesting, as they open up new perspectives for autoimmune diseases which, until now, have been poorly treated.

Chronic inflammation of the eye, uveitis, can be caused by an autoimmune disease. Eventually, immune cells destroy certain structures in the eye, impairing vision. The most recent study on uveitis came out in 2019. It is based on mice suffering from chronic inflammation of the eye. These mice consumed EGCG diluted in water, at levels close to green tea infusions and pure green tea extracts. The extracts and EGCG were able to reduce the dilation of retinal blood vessels, which could limit the passage of immune cells into already inflamed areas. Moreover, pure green tea extracts were more effective than EGCG in reducing pro-inflammatory and pro-immunity cytokines. One of the observed consequences would be a better recovery of vision through consumption of green tea extract, with reduced retinal oedema.