What mechanism causes the symptoms of a histamine-mediated allergic reaction?

The symptoms of a histamine-mediated allergic reaction primarily arise from the action of histamine on various tissues in the body. Histamine's primary roles include causing vasodilation and increasing vascular permeability.

When histamine is released, typically from mast cells and basophils during an allergic response, it binds to histamine receptors on the surface of endothelial cells in blood vessels. This binding leads to an increase in the permeability of the vascular walls, allowing fluid and immune cells to exit the bloodstream and enter surrounding tissues. This process contributes to the hallmark symptoms of an allergic reaction, such as swelling, redness, and itching.

Additionally, the vasodilation caused by histamine leads to an increase in blood flow to the affected areas, which can also result in warmth and redness. These combined actions of increased vascular permeability and vasodilation are central in the development of allergic responses, such as those seen in conditions like hay fever or anaphylaxis.

The other options do not accurately describe the primary mechanism of action for histamine in allergic reactions. While histamine can affect heart rate, blood pressure, glucose metabolism, and neurotransmitter release, these effects are not the main contributors to the typical symptoms seen in allergic reactions.