Histamine toxicity occurs when bacteria in warm, stored fish convert histidine to histamine. Cooking or gut processes do not reduce histamine levels. Consuming such fish introduces histamine into the bloodstream, causing allergic-like symptoms such as flushing, hypotension, swelling, and abdominal cramps. Pathophysiology of Problem 2, Histamine Toxicity. Bacteria in warm, stored fish convert histamine to histamine. Increased histamine levels remain unaffected by cooking or gut processes. Consumed fish introduces histamine into bloodstream. Histamine binds to receptors triggering allergic-like response, symptoms, vasodilation, flushing, hypotension, increased capillary permeability, swelling, hives, smooth muscle contraction, abdominal cramps.