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Why does histamine increase the permeability of blood vessels?

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antihistamines, including Benadryl — antihistamine Antihistamines such as Benadryl (diphenhydramine) and Claritin (loratadine) can be purchased without prescriptions. Both of those agents act by blocking H₁ receptors. However, whereas Benadryl binds to those receptors in the central nervous system (CNS), causing drowsiness, Claritin does not readily enter the CNS and thus does not normally cause drowsiness.

Histamine increases the permeability of blood vessels as a critical component of the body's immune response to injury, infection, or allergic reaction.12

When tissues are damaged, mast cells release histamine, which causes local blood vessels to dilate and their walls to become more permeable. This process allows essential components of the bloodstream—specifically white blood cells (leukocytes) and plasma proteins—to leak out of the vessels and migrate into the surrounding tissue spaces.12

By facilitating this movement, histamine enables the immune system to effectively reach the site of injury or infection to fight pathogens, nourish the area, and begin the healing process. While this mechanism is vital for defense, it also results in the formation of edema (swelling) and the redness often associated with inflammation.12