Here's a explanation that the medical community is starting to see as a possible reason for the rise in allergies in many 1st world countries... First, a little background for allergic reactions. Allergies are caused by a reaction to an antigen by antibodies in the bloodstream. Antigens are substances from outside the body that can make it into the bloodstream and if there are no antibodies against it, no harm, no foul. However, if there are antibodies that have been produced by your body that target that particular antigen, then we have issues.
Our body produces antibodies as a natural immune response to foreign attackers. So, when you get a cold, your body recognizes it as something that shouldn't be there and makes antibodies that target the cold antigen and label it for destruction by the immune system. You have tons of antibodies in your blood all the time, which helps keep you from getting sick from every little germ you come in contact with... it's actually fascinating how it all works.
So, think of antigens as little colored baseball's floating in your body and the antibodies are colored baseball gloves that can only catch the baseballs that are the same color. Blue baseballs are caught by blue gloves, red baseballs by red gloves, etc... you get the picture.
Now, to the crux of the issue, all of these antibodies that cause allergic reactions are made up of IgE antibodies. When you have anaphylaxis (swelling, closing of throat, etc), the IgE antibodies are binding to the allergen baseball and creating rapid downstream effects. These IgE antibodies also have another function in the body: it has been used since the beginning of time to help your body fight off worm infestations. So, all those nasty parasites you can pick up like Schistosoma, Trichinella or Plasmodium (Malaria), are fought off by IgE.
When was the last time you heard of a U.S. citizen getting Malaria? What about a worm infection by Schistosoma called “schistosomiasis”? Hint: you don’t have these much at all anymore in developed countries because we have medications that have eradicated them from our bodies and water treatment that has eradicated it from our drinking water, and food handling mandates that have eradicated it from our cooked food supply. Now, all of those IgE antibodies that have been floating around in your blood have nothing to do. So, they start looking for new antigens to attack and the low hanging fruit is some of the most common things that we eat like peanuts, wheat, olives, etc.
So, one theory for all of these newer allergies relates to the improved living conditions that we enjoy in the United States, worm infestation free.