I've had a lot of problems with what is known as a "pseudoallergy", mostly triggered by food colorings, dyes, and certain foods. I used to have constant issues with hives that would get triggered by heat, friction, and pressure, but they wouldn't show up until days after exposure to the problems and were always worse when I was sick or menstrual.

A true allergy is when your immune system triggers an IgE reaction to an allergen, releasing loads of histamine into your body. Pseudoallergy happens when you have an impaired ability to metabolize and break down histamine, which could originate from your own immune system or could originate from ingested food/drugs. The symptoms are very similar to a traditional allergic reaction, such as hives, anaphylaxis, asthma, dermatographism, etc. However, the timing can be very unusual, happening hours to days later. Also, because it's related to an accumulation of histamine, it's dosage dependent over time. A single megadose of something can cause a reaction, or smaller doses accumulating over days/weeks may cause it. You will also score "negative" (not allergic) to the offending substances on a skin-prick test, as the skin-prick measures true IgE allergy.

Many foods contain natural histamine from fermentation/preservation. Other foods trigger a non-allergic release of histamine in the body by cleaving the bond between histamine and its storage protein. Some foods and chemicals mimic histamine, binding to histamine receptors in the body. Still more foods (and many medications and additives) impair deamine oxidase, which is the enzyme that breaks down histamine in the body. Also, certain nutritional deficiencies (notably B6, vit C, and copper) can cause insufficient supplies of DAO.

This article on histamine intolerance is an excellent summary.