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Summer is here (YEAH) and everyone is going outside to enjoy the sun. We all know the risk of the sun in terms of cancer, but I wondered what does the sun do to our immune system? Is the sun good or bad for our immune cells?
Sunburn is inflammation
If you sit in the sun for too long UV-light (UV-B to be exact) damages the cells in your skin. The light changes the conformation of a molecule within the cells called RNA. Upon this, RNA is expelled from the skin cells, where it activates neighbouring cells as well as the innate immune system via so called Toll-like receptors (TLR3). Infiltrating immune cells and messengers such as cytokines then cause the redness, swelling and soreness we all know (and hate) as sunburn.
Why all this you may ask – it’s a safety net! UV light does not only damage RNA, it also effects DNA, the molecule that encodes all our genetic information. Damage in the DNA can lead to abnormal cell growth – cancer! So by activating the immune system skin cells signal that something is not ok, that they have been damaged and should be destroyed to protect the body from cancer formation.
In the long run, however, sun exposure suppresses the immune system – which in some people leads to cold sores after an extensive sunbath. It’s also why inflammatory autoimmune skin diseases such as psoriasis, where the immune system attacks the body rather than a pathogen, are treated with UV light. The light exposure dampens the immune response leading to an improvement of the symptoms.
Sun or no sun – what is good for the immune system?
One link between sun exposure and immune suppression is vitamin D. The precursor 7DHC is produced in the skin and can be processed into vitamin D by UVB light. The “sunshine vitamin” has immunosuppressive properties, such as inhibiting B and T cell proliferation and depressing the activity of innate immune cells in the skin (called Langerhans cells). In contrast vitamin D supports the production of anti-microbial molecules, which can kill pathogens – so it is not just immunosuppressive vitamin D can also help against infections via an “antibiotic” effects.
Interestingly, a link between vitamin D deficiency and autoimmune disease has been described. For example, multiple sclerosis, an autoimmune disease attacking neurones in the brain, has a higher incidence rate the further away from the equator you get. Similar data is available for diabetes and a clinical trial given children vitamin D supplements during infancy found that these children had a nearly 90% lower risk of developing type 1 diabetes than those who did not receive supplements. So it appears constant exposure with sunlight or vitamin D supplements can help prevent autoimmune disease.
While immunosuppression is a good thing for autoimmune diseases, a dampened immune response to pathogens after a sunbath seems like something we do not want! But don’t fret just yet – scientists actually suggest that sun exposure and therefore higher vitamin D levels could also help against colds and the flu. It is thought that higher levels of vitamin D leads to an increased production of anti-microbial peptides by innate immune cells. In addition the immunosuppressive function of the vitamin reduces the amount of cytokines and innate immune cell activation upon infection leading to weaker symptoms in the patients. Indeed, flu epidemics are more common in the winter months, where there is less light and people have lower levels of vitamin D. So supplements or light therapy might be a way around the flu. A recent trial in Japan supports this idea, showing that vitamin D supplements given during the winter reduces the incidence rate of flu in school children.
So, what did we learn? It appears constant sun exposure is good for us, as it protects from autoimmunity and helps boost the production of anti-microbial molecules. The emphasis being on constant, short sun exposure. As little as 15 minutes sun a day is enough to produce the amount of vitamin D needed per day. And you cannot store vitamins, so 2 days in the bright sun at the lido will not build up a storage of vitamin D for the winter. It will only give you a sunburn and increase your risk of cancer! In conclusion moving south to get sun exposure in the winter would be the best way to go! 🙂