Apothecary
An Apothecary is a Space Marine specialist in Warhammer 40,000, combining the roles of medic and geneticist. The senior Apothecary is known as the Master of the Apothecarion (the fortress-monastery's medical center) or Chief Apothecary. They are usually armed with standard Space Marine gear and a Narthecium.
Since Space Marines have undergone genetic augmentations to survive nearly anything, an Apothecary's real battlefield roles involve three jobs: stabilization, mercy killing, and extraction. While Space Marines can endure damn near anything except the most serious injuries, Apothecaries frequently need to stabilize them long enough for their gene-seed to start up and heal them. If a Marine has been too badly injured to be saved, then the Apothecary applies "the Emperor's Peace," a mercy kill by driving a Reductor (a sharpened needle) into the Marine's skull to quickly end his pain (although some Apothecaries use bolt pistols, for some reason). If a Marine has died, it is the Apothecary's duty to extract the progenoids from his dead brother. Off the battlefield, they also implant Space Marine Initiates with the extracted progenoids, and make sure Battle-Brothers don't have an nasty mutations.
Fun fact: back in the days of Rogue Trader, they actually were called Medics for a while, but that quickly changed.
Sanguinary Priest
The Blood Angels are a little weird, so they call their Apothecaries "Sanguinary Priests." There's a lot of weird blood rituals they do.
Also, they LITERALLY carry the blood of Sanguinius, since they're injected with a sample of it when they become Sanguinary Priests. This means they are the prettiest Marines, as they look the most like Sanguinius.