What histamine is
Histamine is the molecule your body releases during an allergic reaction. Antihistamines are named for it. It causes flushing, headaches, a runny nose, hives, and cramps. In some people it speeds up the heart.
Histamine also forms in food. Protein-rich foods contain the amino acid histidine. Certain bacteria carry an enzyme that converts histidine into histamine. Give those bacteria a protein source and some time and histamine accumulates. A fillet of fish. A pot of curry. A wedge of aged cheese. The longer the food sits, the more histamine builds up.
Reheating will not fix it
The bacteria that make histamine die when you cook. Histamine does not. It is a stable molecule that survives boiling, frying, canning, and the microwave. Once histamine has formed in a food, it stays. You can reheat yesterday's curry to any temperature and it still carries every microgram that built up in the fridge. The reheat was never the problem. The storage time was.
This is also why aged and fermented foods are high in histamine by design. Aged cheese, cured salami, sauerkraut, soy sauce, fish sauce, and wine all depend on time and bacteria. Leftovers are accidental fermentation on a shorter clock.
Freeze, do not fridge
Temperature controls how fast the bacteria work. In the fridge, histamine production slows but keeps creeping up over days. In the freezer it effectively stops, because the bacteria cannot function in a deep freeze.
So the move is simple and a little counterintuitive. If you are not eating the leftovers tomorrow, do not leave them in the fridge to use up later in the week. Portion them and freeze them the day you cook. Freezing on the day locks histamine at its lowest point. Thaw and eat in one go rather than letting a thawed batch sit for days.
Intolerance is not allergy
People conflate these two constantly. A food allergy is the immune system treating a harmless protein as a threat. It can be severe, even anaphylactic, and a tiny amount sets it off.
Histamine intolerance is a capacity problem instead. The gut makes an enzyme called diamine oxidase, or DAO, whose job is to break down histamine from food before it reaches the bloodstream. When a meal's histamine load is higher than your DAO can clear, the excess spills over and you get symptoms. That is why the reactions are so inconsistent. The same dish can be fine fresh and rough as leftovers, fine on a calm day and bad after wine and cheese. The outcome depends on the total load and on how much DAO you have that day.