I don't know anything much about FIV but I do know a lot about histamine, as I suffer from so-called histamine intolerance, or as doctors like to call it idiopathic urticaria. I say so-called because we do know what causes it and it's not an intolerance it's an excess of histamine relative to the body's ability to process it. This can be in the form of trigger-happy mast cells or not enough DAO and/or other enzymes that process the excess, or commonly a combination of the two.
Firstly antihistamines are blockers, they stop the histamine molecules binding to histamine receptors. They leave the excess histamine free floating and do not help to eliminate it. So they help short term but long term the body detects that not enough histamine is reaching receptors and produces more, needing higher and higher doses to prevent symptoms.
Secondly histamine develops in aged food and is not destroyed by cooking, it's an extremely resilient molecule. For humans a low histamine diet is all fresh with no canned, dried, preserved, pickled or otherwise treated foods. Frozen is fine, as freezing suspends the development of histamine but does not destroy it. Not relevant to cats is that certain foods themselves contain histamine or trigger histamine production, things like tomatoes, strawberries, alcohol, chocolate - things a cat would never eat. But fish develops histamine extremely quickly and for humans should be eaten within a couple of hours of being caught before histamine levels get too high.
This is very likely relevant to cats and although I'm not a raw-feeding fan myself I would expect a real food diet to make a big difference for a cat, compared to a diet of canned and/or dried food. Certainly worth a shot before keeping a cat on high dose antihistamines long term.
For more details on histamine intolerance and diet:
https://www.histaminintoleranz.ch/en/introduction.html
