Keep in mind the explanations provided below are significantly simplified for lay public understanding. One can go into cytokines, Th1 and Th2 responses, and other techno-jargon, but not here.
Perhaps the most popular explanation is the "Hygiene Hypothesis" which mainly states that major improvements in public health and medical technology have led to a cleaner home environment with less infectious and parasitic disease burden, resulting in less stimulation of the immune system and a consequent predisposition to allergic disease.
This hypothesis helps to explain why 3rd world nations which have poor sanitation and greater microbial/parasitic diseases do not suffer nearly as much allergic problems as well-developed nations. Basically, the dirtier and more disease ridden people grow up in, the less allergies they have because the immune system is preoccupied with fighting "real" infectious problems. In the absence of such immune stimulation, the immune system essentially gets "bored" and starts treating normal things like pollen as an infection that needs to be eliminated.
The alternative explanation is the "Hapten Hypothesis." Before explaining further how this theory works, first a little immunology instruction. Haptens are very small molecules which are able to easily pass through skin and mucus membranes with ease and by themselves cause absolutely no problems whether immune or allergy. However, when such haptens attach to a carrier protein, it may elicit a profound allergic immune response. Keep in mind that the carrier protein itself may also not elicit an immune response. It is the combination of hapten and carrier protein together that causes the problem.
According to the Hapten Hypothesis, persistent low-grade exposure to environmental haptens via the skin and oral routes at key times during human development – namely, pregnancy and the first year of life – can lead to allergy development. Dietary hapten intake may interfere with oral immune tolerance mechanisms while repeated skin exposure to haptens may promote elevated allergy responses.
What are some of these oral and skin haptens?
• Drugs (antibiotics and others)
• Industrial chemicals
• Food emulsifiers
• Food colorants
• Flavour enhancers
Arguments promoting the Hapten Hypothesis over the Hygiene Hypothesis goes as follows:
#1. The biggest reduction in infections came at the end of the 19th century with improvements in sanitation and nutrition – not in the second half of the 20th century, when the greatest increase in allergic disease began – so why didn't they occur together??? However, significant exposure of haptens to the human population on a recurrent basis occurred on a major scale during the 20th century.
#2. The Hygiene Hypothesis states that our immune system does not get stimulated enough by infections. But technically, that's not true... Vaccination programs are the deliberate exposure of our immune system to a whole host of diseases including polio, tetanus, diphtheria, and measles, all by the age of 1 year. Disease burden is not a factor according to the Hapten Hypothesis.
#3. Studies have repeatedly shown that respiratory infections are associated with the development of allergic disease... but according to the Hygiene Hypothesis, the opposite should occur. According to the Hapten Hypothesis, drugs like antibiotics are haptens and can actually promote allergy development.
So is there a definitive answer here?
Unfortunately not, though there are very smart people working on this.
My hunch is that the final answer may be a combination of both hypothesis.
At the very least, there really may be something to the organic movement which promotes eating/drinking foods that contain no pesticides, preservatives, or chemicals and the use of skin products which also contain only "organic" ingredients.
However, keep in mind that the BIGGEST impact towards the prevention of allergy development are actions taken while a woman is pregnant through the infant's first year of life. After that, interventions may be too little, too late according to the Hapten Hypothesis.
I personally like the Environmental Working Group website to check if certain products are "organic" or not because I certainly don't have the time to check every consumer product myself.
When it comes to food, try to make all baby food from scratch. Next best would be to purchase baby food that is sold refrigerated (avoid baby food sold on regular shelves).
Does hapten exposure predispose to atopic disease? The hapten-atopy hypothesis. Trends Immunol. 2009 Feb;30(2):67-74. doi: 10.1016/j.it.2008.11.006. Epub 2009 Jan 8.
Food-provoked eczema: A hypothesis on the possible role of systemic contact allergy to haptens present in both cosmetics and foods. Estetol Med Kosmetol 2011; 1(1): 35-40. DOI: http://dx.doi.org/10.14320/EMK.2011.006
The importance of hapten-protein complex formation in the development of drug allergy. Current Opinion in Allergy and Clinical Immunology 2014 Jun 17;
Immunological mechanisms in allergic contact dermatitis. Curr Opin Allergy Clin Immunol. 2015 Jan 21. [Epub ahead of print]