(“Hadas” by Jaime Ibarra. This picture was the inspiration for the following article.)
The oxygen transport of Faerie (Fata spp.) blood is accomplished via hemocyanin, rather than hemoglobin as in humans (Homo spp.). As a result, their oxygenated blood is bright blue, which results in a pale bluish cast to their skin, as opposed to the pinkish hue of humans. Faerie blood dries first to a reddish color as the copper oxidizes to cuprous (I) oxide, then to black as these oxides further degrade to cupric (II) oxide.
It is speculated that this is one source of the danger posed to the Fae by iron implements. As hemocyanin is less efficient at oxygen transport as hemoglobin, iron contamination may interfere with oxygen transport in Faerie blood, acting as a poison which can pose a great threat to Faerie health.
Strangely, this difference does not seem to pose any problem for faerie/human interbreeding, nor do the differences in DNA between the species seem to pose any obstacle. Although most such unions do not result in a viable pregnancy, there have been cases reported in the literature where pregnancies have been brought to full term without complication.
Children resulting from these unions are generally known as “Halflings”. Their blood exhibits a combination of hemoglobin and hemocyanin, and appears a vivid violet hue when oxygenated. One advantage of this hybridization of the blood is that these children are able to receive donated blood from both faerie and human sources, although a mix of blood is more ideal in the event of the necessity for transfusion.
It is also speculated that the copper base of Faerie blood is part of the reason why the Fae seem to be more adept than humans, on average, at manipulating magickal forces. It is thought that the higher magnetic permeability of iron as compared to copper causes a certain interference with magic. For the same reason, iron implements tend to interfere with magic, and this is another possible reason why the Fae avoid iron.
It is possible that the differences in oxygen transport are what result in the relatively long life of most Faeries. Because their blood transfers less oxygen, their bodies are exposed to far less of the free radicals which are thought to be a primary cause of aging among humans.