A person can become allergic to any organic substance. Allergies are caused by a misdirected immune response--that part of the immune system is meant to fight off parasitic infections. In our overly clean 1rst world environment, we rarely if ever come into contact with internal parasites, so our immune system can easily become misdirected, and attack something harmless.
This is why folks who live in 3rd world countries virtually never have allergies--because they have parasites. Parasites help regulate the immune system so it doesn't overreact to harmless substances, or attack itself.
This is such an important revelation, that there is a new treatment being explored for autoimmune diseases such as rheumatoid arthritis.
http://www.hygienehypothesis.com/worm-therapy.html
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