What are the causes of food allergy?

Food allergy happens when the body's immune system responds in a negative way to proteins in foods.  The immune system’s usual job is to protects us from germs, but in this case the response is not helpful.  There are two ways that the immune system might cause food allergy.  One way is by creating a protein called IgE antibody that recognizes the food protein and triggers a swift allergic response.  The symptoms of this kind of food allergy usually happen soon after the food is eaten.  Typical symptoms include hives that look like mosquito bites, swelling and itching of the skin; gut symptoms such as nausea, vomiting and diarrhea; breathing symptoms that could be severe including trouble breathing; and potentially blood circulation can be compromised leading to dizziness, confusion or fainting.  In its most severe form, anaphylaxis, we experience a severe allergic reaction that can be fatal.  Another way the immune system causes problems is more insidious.  Different immune cells might respond against food protein in a persistent and chronic way leading to persistent skin rashes or gut problems.  These are often more difficult to diagnose because there are many causes of rashes and gut problems.  We do not entirely understand why our immune system might cause these attacks.  There are theories that the parts of the immune system that cause these problems are also the parts of the immune system that usually fight infections with worms or parasites.  We have done a lot to protect our immune system from these and many other types of infections through clean living and some people think that this has left our immune system essentially "looking for a fight".  Therefore, our immune system might become misdirected and attack foods as well as other things that cause allergy, such as pollens, animal dander, mold, etc..  There is some evidence that food allergy may be increasing, and there are a variety of theories about why this is so, but no one has clear answers.  Theories include: the way that our food is processed potentially making it more easily seen by the immune system, aspects of our diet in general, such as the types of fats we eat, also vitamins in our diet, the cleanliness hypothesis mentioned previously, and perhaps introducing foods to babies either too soon or too late for the immune system to act correctly.