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In many places I have heard that living too clean weakens your immune system. For example, a Daily Mail article says:

Evidence is growing that dirt and germs can protect against disease - and that our indoor-based, ultra-clean lifestyles are bad for our health.

My doubts are:

  1. Is it true for both adult and a child?
  2. If yes then what is the medical-scientific basis of this claim?

From my experience I have seen that people living in very dirty places, eating street foods regularly, can adapt to unhygienic environments and foods much easily than people living in more clean environments from birth.

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1) Yes.

2) Your immune system's memory response and capacity for leniency towards non-self antigens.

Here's a nice, comprehensive look that contrasts cleanliness versus pollutants:

D. Diaz-Sanchez Pollution and the immune response: atopic diseases – are we too dirty or too clean?

I can quickly summarize some of the basics behind the theory (antigen is basically anything that's not part of your body - it can be a virus, a bacteria, or just chemicals it doesn't recognize):

Your immune system has both a nominal response and a memory response. The initial response is made by the Innate immune system, and is responsible for allergies (Mast Cells contain large amounts of Histamine - the primary agent in allergic reactions). Whether or not your immune system responds a little bit, a lot, or just the right amount depends on previous exposures to antigens. Your Innate immune system can learn that some compounds (like pollen) don't require a large response, but only if it's been consistently exposed to pollen before. This is part of the reason why "Exposure Therapy" for allergies can yield success. It's basically repeated exposure to the allergen at higher and higher doses until the immune system stops overreacting.

The Memory Response is one of the most effective ways your body fights off infections, and it's the reason why vaccines are so effective. Once you are exposed to an antigen, your Adaptive immune system retains some white blood cells in the lymphoid tissues. If your body encounters the same antigen again (Chickenpox), it already has an extremely effective responder ready to undergo mitosis to quell the invasion. As you age, your immunological memory expands, to the point that removing the Thymus (the primary source of new T-cells - which are the basis for the Memory Response) becomes less and less of an issue as you age. After about the age of 35-40, the removal of the Thymus barely affects the immune response at all since the person is more or less encountering what they've already seen.

Between the two the argument for the "Living too cleanly" is basically thus:

If you do not encounter a lot of antigens, like in a sterile environment, your body's immune system does not correctly calibrate itself and can overreact when it is exposed to unfamiliar antigens. Also, there's the possibility that if you were to spend several decades away from sources of antigens, that your Memory Response library would be severely lacking - resulting in more severe illnesses which take more of a toll on an elderly person than young person.

I'm not aware of any threshold of cleanliness that there's been agreement upon deciding which level of cleanliness is fine or not. I would think that as long as you get outside often, socialize occasionally, and keep your living areas relatively clutter free you're good. Obviously do not do something like refuse to clean food prep areas under the delusion that it will only make your immune system stronger. It might do that until you catch Salmonella or something much worse, then you're in the hospital paying a few thousand dollars to vomit into a fancy bucket.

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    <nitpick>Its like saying lack of training weakens your body. Technically it does not weaken your immune system but leaves it in a weaker state than it could be with training. And thats what exposure to dirt etc is for the immune system training.</nitpick> +1 tho ;)
    – Stefan
    Aug 30, 2012 at 20:33
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    That's not what I said. I said that a lack of exposure to antigens creates a situation where your immune system doesn't recognize the appropriate response magnitude. Often the immune system overcompensates, resulting in allergies. Sometimes it doesn't respond at all, which is just as dangerous.
    – MCM
    Aug 31, 2012 at 3:05
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    It still does not weaken your immune system. It just leaves it in a state where it does tend to make wrong decisions. Immune system weakening would be for an example HIV where the system is actually losing response capability. In this case it never had it to begin with.
    – Stefan
    Aug 31, 2012 at 12:26
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Yes, for the reasons MCM already stated, HOWEVER...

Vaccinations can teach the immune system a lot of little bit of what it missed learning the natural way by injecting antigens into the body in a controlled process.

There is a school of thought that says that humans in developed societies are developing weak immune systems because they aren't exposed to enough grit and grime. With vaccinations, that is not necessarily true.

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    No, vaccines don't. Vaccines immunize against a specific antigen each and every time. This is why Flu vaccines have to be taken yearly instead of just one Flu vaccine with similar epitopes. Getting all of your shots will not prevent your hayfever. Getting outside often as a kid and teenager will because the antigens are different.
    – MCM
    Aug 31, 2012 at 3:06
  • @MCM, anecdotal, but I grew up in the countryside, spent all my childhood years outside. Only developed hayfever when I moved into a big town during adolescence. Go figure.
    – Benjol
    Aug 31, 2012 at 6:02
  • @MCM - we don't disagree. I didn't say a single vaccine will inject multiple antigens. You can always keep taking vaccines.
    – shtimss1970
    Aug 31, 2012 at 11:33
  • @shtimss1970 the point is that vaccines are more targeted than normal exposure. And all that they do is trick your body into producing antigens that it was not producing prior to the vaccine. This does not replace the normal function of your immune system.
    – Chad
    Aug 31, 2012 at 13:08
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    @shtimss1970 - We're not disagreeing over something particular, but I'm not sure you know what I meant. You can always keep taking vaccines, but there aren't vaccines for 99.99999999999999999% of the stuff your immune system will encounter. They're not a substitute for simply getting dirty. They're made to combat a very specific, often debilitating or deadly disease which you'll encounter once or twice in your life. You encounter some strains of Salmonella on a weekly basis. That's the difference. :)
    – MCM
    Aug 31, 2012 at 13:35

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