PATHOLOGY

Detailed explanation-1: -During an allergic reaction, histamines bind to receptors along the blood vessels, making them expand and become more permeable so white blood cells can attack the pathogen. Antihistamines block this mechanism, in effect narrowing the blood vessels.

Detailed explanation-2: -Intravital imaging showed that histamine dilated vasculature, increased blood flow, while it induced hyperpermeability in venula.

Detailed explanation-3: -How antihistamines work. Antihistamines block the effects of a substance called histamine in your body. Histamine is normally released when your body detects something harmful, such as an infection. It causes blood vessels to expand and the skin to swell, which helps protect the body.

Detailed explanation-4: -Antihistamines suppress the histamine-induced wheal response (swelling) and flare response (vasodilation) by blocking the binding of histamine to its receptors or reducing histamine receptor activity on nerves, vascular smooth muscle, glandular cells, endothelium, and mast cells.

Detailed explanation-5: -Histamine causes constriction of various smooth muscles with the bronchioles being most sensitive to its effects. It also commonly produces vasodilation as a result of effects on vascular smooth muscle. Histamine also has multiple effects on the heart and nervous systems.