IMMUNOLOGY

Detailed explanation-1: -Conclusions: Histamine can produce vasodilation of submucosal arterioles by two distinct mechanisms: activation of vascular H1 receptors resulting in release of nitric oxide from endothelium and activation of H3 receptors on sympathetic nerve terminals resulting in presynaptic inhibition of vasoconstrictor tone.

Detailed explanation-2: -Histamine causes constriction of various smooth muscles with the bronchioles being most sensitive to its effects.

Detailed explanation-3: -Histamine, an important component of immune and inflammatory responses, is produced and released within the affected tissue, eliciting an increase in blood flow by its vasodilatory actions on arterioles and an increase in vascular permeability by its actions on capillaries (8, 29, 51).