Your body has an amazing line of defense with your immune system, which works with a variety of cells that protect your body from the stuff in the environment that finds its way into you.
But first, it has to determine whether or not the substance is harmful. And for that, it has to be recognized by your body's cellular defense system.
So, the images show the main cell, the mast cell, that is responsible for recognizing the harmful stuff. It then also deploys histamines which bind to specific receptors on other cells. One place H1 receptors are found is on blood vessels. When the histamine binds to that receptor, it makes the vessel more "leaky." This is what promotes fluid leaking into the skin, creating localized swelling and HIVES!