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Carl Fredrik Ahl

Why The Immune System Gets Weaker When You Are Cold?

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I'll defer to a biologist here, but my understanding is that the bigger issue is people tend to huddle closer together when it's cold. We spend far more time indoors and in contact when it's cold than we do when it's warm and we're outdoors and farther apart from other humans.

There may be an effect from the body needing to burn calories to stay warm and reallocate energy from strong immune response instead to maintaining a baseline body temperature, but last I heard that is not a very powerful affect is is more of a myth.

Note: We could surely google this to confirm, and I'm not a biologist so concede in advance the above may be mistaken. I look forward to being corrected if that's the case. :)

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I used to ask this question when i was a nurse and got all kinds of answers. Viruses prefer the colder temperatures in noses during winter; immune systems weaken during winter months as part of annual biorhythms; people seal themselves off from the outside world more to stay warm (similar to the huddle together one). Some mix of all the above. I never did find a definitive answer.

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We don't get enough Vitamin D when we can't get outside often (as with colder weather), and that can lead to complications. Also, it's sort of a design flaw that when your nose is stuffy from a cold, you breathe through your mouth, which bypasses your nasal filtering system and allows more viruses and bacteria in. 

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We are talking about different measures here. Weaker immune system is a mechanistically different from increased infection rate. The latter is also dependent on a lot of different external factors. On the tissue level coldness can affect vasoconstriction, another element of respiratory infections. One aspect that is also unrelated to the immune system is dryness in the winter time, which affects mucosal surfaces.

When we talk about the immune system proper, we have to look at factors influencing things on the  (sub)-cellular level. And here things get quite complicated to a degree that I do not expect MDs to have a deep knowledge about.

For starters, all kinds of stressors have the potential to affect cytokine expression and inflammatory signaling in general. Also if locally temperatures are lowered, immune cell responses are significantly altered, However, that link is mostly derived by in vitro experiments and it is not clear how applicable it is to animals. Additional stress such as exercising in cold does effect the lymphatic system, but it is not clear whether the observed changes are actually immune mechanism in any measurable way. In short, we know things change at lower temperature, especially when cold air is inhaled or the body is also stressed with other factors. However, we do have a bit of a knowledge gap in translating that molecular change into actual health outcome. As usual, biology is multifactorial and it is quite difficult to make easy explanations based on what we know.

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