Histamine can have a number of different effects including swelling of body tissues, widening of blood vessels, contraction of smooth muscles, hives, itching, sneezing, and nausea.
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The effect of histamine release by mast cells depends on where in the body the mast cells are located. For example, when mast cells near the eye release histamine, it causes fluid to leave the blood and enter the tissue causing watery eyes.