The short story is that it is due to an over-abundance of red blood cells. More precisely, “detailed evaluation of adult reindeer’s nasal microcirculation revealed similarities with human nasal microvasculature, but also striking differences. Reindeer nasal microcirculation exhibited a highly vascularized nasal mucosa, a red cell-rich nasal septal mucosa, and a microvessel density 25% greater than that of humans. The architecturally distinct nasal microvasculature confers on Rudolph a nose that is red and well adapted to carrying out his duties in extreme temperatures,” according to scientists from the Erasmus Medical Center in Rotterdam in the Netherlands.
Not all scientists agree. Some speculate the red nose could be the result of parasitic infection, the common cold, or even alcohol consumption.