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).
Histamine which niacin helps to produce is a compound that is capable of dilating blood vessels, which in turn improves circulation. This helps blood to circulate to the hands, feet and all parts of the body, which helps to prevent atherosclerosis or what is sometimes called hardening of the arteries.