A friend's daughter, home for the summer and halfway through college, told me over dinner that she's developed allergies to nuts, shellfish, and other foods. Either she didn't have these sensitivities before, or she didn't notice them.
She doesn't have tests from several years ago as a control, but she's always enjoyed a wide variety of foods without incident. So I explained to her the role of Vitamin A and the immune system as it relates to food sensitivity.
Vitamin A becomes Retinoic Acid (RA) in the body, which manages the development of T Cells (key immune cells), the activation when a "threat" is detected, and modulating immune responses when T cells are activated - so it doesn't get out of hand and give you hives every time you see shrimp.
Everything we're exposed to by eating or living in the world is non-self and could or should theoretically provoke a response. Fortunately, the body uses Retinoic Acid to "train" the immune system not to overreact. Besides the general implications for autoimmune disease, this has particular application for any kind of food or environment sensitivity.
Although she's already lining up immunotherapy, I sent her home with Cod Liver Oil as a source of Vitamin A. Because of the time it takes to mature these T cells, several months of adequate to high Vitamin A intake is necessary to dampen the immune response.
I've experienced this firsthand with seasonal allergies. This spring there were a few weeks when I needed herbal antihistamine or Claritin to not have itchy watery eyes. I had been taking a maintenance dose of Vitamin A, but perhaps with nursing it wasn't enough. So I increased my Vitamin A beyond the maintenance dose for a couple of weeks and the itchiness just stopped. Back to the maintenance dose, and no seasonal allergies for the past two or three months.
Hope this help someone dealing with food sensitivities or seasonal allergies!