of, relating to, or of the nature of preference: preferential policies. 2. showing or giving preference: a preferential hiring system. 3. receiving or enjoying preference, as a country in trade relations; favored. Origin of preferential.
Histamine dilates post-capillary venules, activates the endothelium, and increases blood vessel permeability. This leads to local edema (swelling), warmth, redness, and the attraction of other inflammatory cells to the site of release. It also depolarizes nerve endings (leading to itching or pain).
But in an allergic episode, the cells involved are mast cells and basophils. When mast cells and basophils are destroyed, their stores of histamine and other allergy mediators are released into the surrounding tissues and blood. This causes dilation of surface blood vessels and a subsequent drop in blood pressure.