Food contains histamine. The longer the processing time or stoorage time, the more histamine the food contains.
Some people have a genetic mutation that makes them unable or less able to break down histamine in the GI tract because they dont have (or dont have enough of) an enzyme. Other histamine sensitive people have an allergenic immune system disorder.
People who are sensitive to histamine avoid high histamine foods, histamine-liberating foods, and must choose food carefully for its processing and storage. They typically cannot eat leftovers, fish, or meat.