In the video below, is represented a 1mm³ skin cube. It contains a network of nerves, a capillary as well as mast cells.

-with the virtual syringe, we inject an allergenic product
-mast cells release histamine
-the allergic reaction spreads with the histamine
-to parry what seems to be an attack, organism causes a vasodilation (vessels become permeable)
-plasma escapes and an edema forms on the skin surface