Blood

Blood is a specialized bodily fluid that is responsible for both delivering nutrients and oxygen to the cells as well as transporting metabolic waste products away from those cells. Immortals, vampires and hybrids need to drink blood to function. If a vampiric being cannot feed on blood for an extended period of time, they will weaken to the point of desiccation, which will essentially turn them into a statue-like state until someone else feeds them. Blood is also what provides vampiric beings with their supernatural abilities, such as enhanced strength, speed, agility, senses, healing, and mind control, and as a result, the strength of these abilities can be dependent on how recently they've fed; starvation can weaken a vampiric being's powers considerably, whereas someone who has just fed on a significant amount of blood will experience a slight increase in the strength of these powers. Most vampires drink human blood, but there are some who choose to exclusively drink animal blood, such as Stefan Salvatore, or vampire blood, such as Mikael. However, animal blood does not provide the same level of nourishment to vampires as human blood does, which is why most vampires won't even consider it unless they have no other option.
Witches can integrate the use of blood into their magical practice to strengthen certain types of spells, which range from simple and minor locator spells to more powerful and complex curses and hexes whose effects can be so devastating that they can even kill a human or supernatural creature.
== Types of Blood ==
=== Human ===
Feeding on human blood is the final phase in becoming a vampire after a human has died with a vampire's blood in their system and has subsequently reawakened in transition. If an individual chooses not to feed and resists human blood, the transition will not be complete and the individual will die permanently within twenty-four hours. (An example of this is Bill Forbes, who died after failing to drink blood to complete his transition in Bringing Out The Dead). However, the urge for human blood is a crucial aspect of life as a vampire— the bloodlust and hunger becomes as constant as breathing, and every individual must learn to adjust to this new lifestyle in order to maintain self-control and to avoid exposure. 
When a vampire experiences moments of hunger, bloodlust, or any other heightened emotion, their vampiric aspect becomes evident in their facial changes—their eyes become extremely bloodshot, the veins around them darken and protrude, and their fangs become exposed. The majority of vampires drink human blood. Some choose to consume donations from blood banks as a more ethical way to satisfy their needs, while other vampires feed off of willing or compelled individuals. There are also those who go so far as to feed on their victims until their death, though it has been proven that it is not necessary for a vampire to kill in order to feed—it is simply a skill that takes practice to achieve. 
Furthermore, animal blood makes a vampire considerably weaker than those who drink that of humans, though the vampire's advanced age may still give them an advantage in a fight. The latter point was demonstrated to be the case when Damon returned to Mystic Falls; he and Stefan immediately engaged in a fight in which Damon easily bested his younger brother because he had embraced his vampiric ways with open arms, while Stefan stuck to a strict animal-only diet because of the guilt he was burdened with from his past. 
It has also been shown that the blood from witches and werewolves in their human form seems to have the same effect on vampires as human blood, as vampires have been seen to feed on either species with no negative physical effects.
=== Animal ===
Some vampires (such as Stefan Salvatore) choose to feed on animal blood rather than human blood. Animal blood does not provide the same level of nourishment to vampires as human blood does, and vampires who drink animal blood have decreased strength, decreases speed, decreased healing abilities and a lower tolerance to pain as compared to vampires who feed on human blood. Since it is stated that to complete a transition into a vampire, one must drink human blood, it can be presumed that feeding on animal blood will not suffice to complete the transition into a vampire. This assumption is supported by the fact that there has been no vampire on either series who has completed their transition by feeding on blood from any non-human source. Due to the fact that animal blood is both unappetizing, physically unsatisfying, and not conducive to maintaining vampiric powers to their highest level, most vampires will not even consider drinking it unless they have no other choice, such as being starved in an area where there are no humans or blood banks nearby.
=== Vampire ===
Vampire blood is necessary in the process of turning a human into a vampire. The human must have vampire blood in their system at the time of their death in order to begin the transition. However, vampire blood cannot turn a human into a vampire if the human is already dead when it is ingested, nor can it be used to heal someone from a fatal wound if they have already died. As soon as a human has fully transitioned, their human blood becomes infused with the particular magic that reanimates, sustains and operates their undead body, thereby turning it into vampire blood. The reanimated body will thereafter continue to produce vampire blood automatically and continuously in the same manner a human body continuously produces human blood.
Vampire blood also has strong regenerative properties, and when ingested by a human(including Witches and werewolves who are untriggered or in human form), it will heal them of almost any wound and the majority of illnesses within seconds. It was also revealed in Grayson Gilbert's medical journal that he used vampire blood to cure Megan King of a heart condition when she was a child during his tenure in the Augustine Society. 
Vampire blood cannot cure cancer, however, most likely since it works by increasing the speed at which cells regenerate to heal wounds, whereas cancer cells are biologically immortal (that is to say, they don't die from old age like normal, healthy cells, and cancer cells can only be "killed" by methods such as surgery, chemotherapy, radiation, etc). As a result of the physiology of cancer, the exposure of cancer cells to vampire blood creates many problems due to their out-of-control growth.
An exhaustive list of which conditions, diseases, and illnesses can and cannot be treated/cured by vampire blood is currently unknown. In Dead Man on Campus, Damon Salvatore injected Wes Maxfield with extremely serious and oftentimes fatal diseases such as rabies and ebola to torture him, and he was later healed with vampire blood with no ill effects shown afterwards. However, both Enzo and Lily Salvatore have stated that they died of consumption, now known as tuberculosis, with vampire blood in their systems, which allowed them to awaken as vampires who were completely free of the effects of the disease. Whether this means that there are at least some pathogens that vampire blood cannot cure or whether this is just a case of retroactive continuity has yet to be determined. 
It has also been shown that vampire blood will have the same regenerative effects if injected directly into the blood stream rather than ingested orally, as evidenced in She's Come Undone, when Caroline Forbes injected her mother with her blood via a syringe when Liz was unconscious and therefore unable to drink it—her blood still fully healed her mother's injuries. Bill Forbes was also healed with vampire blood using the same method by Dr. Meredith Fell in The Ties That Bind after he was attacked by Tyler Lockwood in his werewolf form.
Vampires are capable of feeding on the blood of other vampires for sustenance, as demonstrated by Mikael and Mohinder. However, it was implied by both of these highly disciplined vampires that it takes time and practice to learn to live exclusively on vampire blood; in most other instances, vampires will only feed on other vampires as a last resort similarly to their stance on animal blood, otherwise it either never crosses their minds anyway or they view doing so as somewhat strange or socially irregular. Mikael has been known as one of the most powerful vampires in history, if not the most powerful, but it is unknown whether his diet of vampire blood caused his increased ferocity, or if it was simply Mikael's personality combined with the fact that he was a thousand-year-old Original Vampire that made him so formidable. According to Damon Salvatore, vampire blood has a distinctive "smooth" aftertaste that human blood does not.
Meanwhile, Augustine Vampires such as Jesse have a natural desire to feed off of vampire blood instead of human blood due to either being conditioned to crave it or by being injected with the "Ripper compound." In the case of Augustine vampires, a diet of vampire blood can actually significantly increase their physical strength, as the newborn vampire Jesse was easily able to overpower Damon Salvatore, who was over 150 years older than him.
While werewolves don't require vampire blood to survive, it has been confirmed that werewolves transformed under the full moon do, can, and will eat vampires, as they have evolved to hunt them as their main prey as a result of the centuries-long war between the two species.
According to Damon and Stefan, the sharing of blood between vampires is a very intimate and personal experience. 
==== Vampire Healing ====
* Klaus
** Elena
** Damon
** Caroline
** Katerina
** Stefan
** Diego
** Marcel
** Elijah
** Rebekah
** Lucien
** Various other people
* Elijah
** Cami
** Freya
* Rebekah
** April
** Aurora
* Stefan
** Elena
** Meredith
** Bonnie
** April
** Various other people	
* Elena
** Jeremy
** April
** Liv Parker
** Josette Parker
* Damon
** Elena
** Vicki
** Caroline
** Jeremy
** Various other people
* Caroline
** April
** Matt
** Elizabeth
** Jesse
** Various other people
* Hayley
** Cami
** Freya
=== Witch ===
Although generally, witch blood contains no properties that make it different than human blood to a vampire, it has many additional uses throughout the witch community, including what has come to be called "blood magic." While the origins of this practice remain a mystery, it can be traced back to at least as early as the 1st century BCE during the time that Qetsiyah and Silas lived in Ancient Greece. 
: Main article: Spells and Rituals
At the most basic level, blood magic can be useful to enhance the abilities of minor spells, while at a more complex level, it can be used to curse, hex or even kill humans and supernatural creatures alike. For example, blood is most frequently incorporated into locator and linking spells, strengthening the accuracy of the spell by using blood that has a connection to the person who is being located or linked. This has been demonstrated on numerous occasions by witches, from novices to those who are extremely experienced in witchcraft. With regards to locator spells, Bonnie frequently used blood within her locator spells to locate her targets, such as Jeremy giving her his blood so that Bonnie could locate Elena, since they are biologically cousins and thus bound by their paternal bloodline. Likewise, the powerful witch Dahlia needed Klaus' blood to locate his baby daughter Hope. 
In terms of linking spells, Bonnie has donated her blood to Damon, Stefan, and Klaus in order to link herself to the others so that she could perform the desiccation spell on Alaric, though it was ultimately used on Klaus; Olivia Parker used this method to a similar effect by giving Elena and Damon her blood so that she would be linked to them, allowing her to be able to locate them and pull them out of the 1994 Prison World through it. Joshua Parker once incorporated his blood and a photo of Jo in order to kill her, even from 3,000 miles away, once he learned that Kai could be freed from the Prison World.
In No More Heartbreaks, Vincent Griffith mentioned the use of blood magic as he and Freya attempted to heal and cure Cami from Lucien's deadly bite. Blood has also been incorporated into transference spells (such as transferring the status of the Anchor to The Other Side and Huntress to Bonnie), possession spells (Maddox and Greta Martin used Klaus' blood to place his spirit back into his own body after possessing Alaric), and boundary and sealing spells. Such boundary and sealing spells are a specialty among the Bennett witches, which dates back to Qetsiyah's era, but the practice is also used by other witches, such as Mary-Alice Claire when she sealed Kol's Playhouse so that it could only be opened by a Claire witch. In general, blood is incredibly useful in spells of this variety, as it essentially acts as a lock on the spell to ensure that it can only be broken by another witch of their bloodline.
Witch blood can even be used to break certain spells, as proven in Dance Back from the Grave when Elijah and Hayley used Hayley's blood (which she shared with her witch-vampire-werewolf hybrid baby Hope while she was in utero) to break the boundary spell cast by Papa Tunde to trap Rebekah's body while he channeled her power.
==== Bennett Blood ====
Members of the Bennett bloodline have a tradition of using their blood to bind powerful spells. This process can bind the spell in a variety of ways, though it typically requires the blood of an active Bennett witch to break, reverse, or remove the spell, or, in the case of enchanted objects, Bennett blood is required to use or open it. For example, Bennett witches were necessary to unseal the coffin that contained Esther Mikaelson's magically-preserved body, since the sealing spell had been cast by a Bennett witch named Ayana; this spell required both Bonnie and Abby Bennett to use their magic and blood to unseal it. Bennett blood and spells were also used by the Gemini Coven to create two separate Prison Worlds in 1903 and 1994, and as a result, Bennett blood was necessary to activate the Ascendant to travel between these "pocket dimensions" and the living world. It was also Bennett blood and magic that sealed the vault in The Armory, which meant that Bennett blood and magic was needed to open it and reseal it afterward. 
==== Qetsiyah's Blood ====
Qetsiyah's blood is a powerful ingredient in spells due to the immense magical power she possessed when she was alive. Bonnie used Silas' Tombstone, which contained a large amount of Qetsiyah's calcified blood, in order to channel the power with Qetsiyah's blood to temporarily lower The Veil to The Other Side. Using Qetsiyah's blood, Bonnie was saved from having to wait until the full moon to do the spell. Qetsiyah's blood, in addition to the power from the Expression Triangle, was more than enough power to successfully complete this powerful spell. While Bonnie was trapped in the 1994 Prison World without magic, she traveled back to the island to get the Prison World's version of Silas' headstone in order to channel Qetsiyah's blood as a source of magic. With Qetsiyah's magical blood, the Ascendant, and the combination of a solar eclipse and the Aurora Borealis that resulted from the colliding Prison Worlds, Bonnie was able to portal her way back to the living world in Let Her Go.
==== Mikaelson Blood ====
The blood of a Mikaelson witch can be used by other Witches as a catalyst to replicate Esther's modified version of Qetsiyah's original immortality spell that she used to create the Original vampires; this was proven when the Ancestors forced Vincent Griffith to use Freya Mikaelson's blood in order to create an "upgraded" version of the spell to turn Lucien Castle (and later, Marcel Gerard) into Beasts that are more powerful than the Originals. However, since the majority of the Mikaelsons are undead vampires, the only remaining sources of Mikaelson witch blood are from Freya and Hope.
=== Hybrid ===
==== Niklaus Mikaelson ====
Klaus is the only being in the world whose blood can cure vampire of the lethal effects of a werewolf bite. After a bitten vampire ingests Klaus' blood, he or she is immediately healed of the bite wound "as though it never happened," in Klaus' words. Additionally, Klaus is one of only two beings on earth whose blood can turn a werewolf into a hybrid, assuming that he has doppelgänger blood in order to complete their transition. The other being is his daughter, Hope. However, Klaus' blood cannot heal a vampire who has been bitten by a Beast.
==== Hope Mikaelson ====
Hope's blood can be used to turn werewolves into hybrids, though this process also required the hybrid-in-transition to feed on her blood again to complete the transformation. Like her father, her blood can cure a werewolf bites, which was demonstrated when she healed MG after being bite by Rafael in The Boy Who Still Has a Lot of Good to Do. Also like Klaus, her blood is not effective in curing the bite of a Beast. In addition to these qualities, Hope's blood can also act as a potent binding agent and can be used to break complex spells cast by other witches as a result of her witch heritage. Unique to Hope, as her status of being one of Nature's loopholes, her blood is also a curative to Malivore mud bullets, weaponized by Triad scientists; this was proven when her blood healed Josie's wound in There's Always a Loophole. In See You On The Other Side, as a fully activated tribrid, her blood is lethal to Malivore.
=== Doppelgänger ===
Human doppelgänger blood was needed in the spell that transformed Mikael and his children into the Original vampires, and was also a required ingredient in the sacrifice ritual necessary to break the Hybrid Curse that was placed on Klaus Mikaelson in the early 11th century. 
Human doppelgänger blood was also a necessary component for Klaus, the Original hybrid, to turn other triggered werewolves into hybrids. Instead of feeding on regular human blood after awakening during the transition period like vampires have to, these hybrids instead needed feed on human doppelgänger blood. Klaus used Elena's blood to break his curse, and then continued to use it in order to sire his hybrids, including Tyler Lockwood, as he was under the impression that she was the only human doppelgänger in existence. However, after Elena was turned into a vampire, her blood could no longer be used to sire hybrids. 
The blood of a doppelgänger is also a powerful source of magic for witches to use in spells due to their mystical nature, and their blood has been used on many different occasions in order to serve as a binding agent or a source of power for extremely powerful spells. For example, the Travelers also discovered that the blood of the last two remaining doppelgängers from each bloodline (Stefan from Silas' bloodline, and Elena from Amara's bloodline) can be mixed together to be used to reverse any non-traditional form of magic. The Travelers intended on using their mixed blood to undo the curse put on their people by the witches, which would slowly undo every spell cast using a non-Traditional Magicsource, including the spells that created The Other Side, the werewolf curse/activated lycanthropy, the spell that created the Originals (as well as every single "living" vampire descended from them) and the daylight rings that protect them. 
==== Magical Purposes ====
* Tatia's blood was used in the adapted version of the Immortality Spell that was used on the Mikaelson family (Mikael, Finn, Elijah, Klaus, Kol, and Rebekah) in order to create the Original vampires.
* Tatia's doppelgänger blood was also used to perform the Hybrid Curse, which was placed on Klaus to keep his werewolf side from manifesting after he triggered his curse shortly after his transformation into a vampire. Elena's blood was used to break this curse over 1,000 years later, as she is a descendant of Tatia and, at the time, was a human doppelgänger.
* Elena's blood was also taken later by Esther, who used her blood to bind a spell which linked all of her children together. Her plan was to cast a spell to make Finn human again, which would make the rest of the siblings human again as well. Then, she planned to kill Finn, which would result in the deaths of all of her children. This plan ultimately failed, because her connection to her source of magic (the entirety of the Bennett bloodline, living and dead) was severed by Abby's death and subsequent transformation into a vampire.
* When her first plan failed, Esther used Elena's blood again to turn Alaric Saltzmaninto an Enhanced Original Vampire who would have superior supernatural abilities that would allow him to finally kill her children and the rest of the vampires once and for all.
* Qetsiyah used the blood of progenitor of the female doppelgänger line, Amara, as well as that of two of her shadows (Elena and Katherine), to transfer the status of Anchor to the Other Side from Amara to Bonnie Bennett.
* Markos mixed the blood of the final two doppelgängers (Stefan and Elena) to do an experiment, by turning Sloan into a vampire. Then, he had her drink the mixed doppelgänger blood, while he performed a minor version of the Purification Spell which stripped away the vampire magic. Unfortunately for her, the blood wasn't a cure for vampirism, as it just removed magic that maintained vampires, which was created by witch Spirit Magic. As a result, Sloan was returned to her previous state before becoming a vampire, which was a human who had her throat slit by Markos. Because the magic that reanimated her body was gone, her throat re-slit itself, and Sloan died once again.
* As a result of Markos' experiment on Sloan, the Travelers learned that the curse that was placed on them could be lifted using the mixed blood of the last remaining pair of doppelgängers. This was later accomplished by the Travelers using the last of the mixed doppelgänger blood to cast the Magic Purification Spell over Mystic Falls.
=== The Cure ===
Once the Cure has been ingested, the blood of the human who took it automatically becomes the Cure itself. The blood retains its ability to make a vampire or immortal human. However, one weakness is that the Cure can be taken from its host by draining the Cure. The Cure has been in six bodies: Katherine, Silas, Amara (all three using the original Cure), and Elena, Stefan and Damon (using the duplicated 1994 Prison World Cure).
=== The Huntress ===
When Rayna was spelled by The Eight Everlastings, Native American shamans, into becoming the Huntress, the Tea of Protection provided her near-complete resistance to any offensive magic that sought to do her harm. Her blood was also given special properties so that if a witch was to ingest her blood, it would completely suppress the witch's magic. Continued exposure to her blood (for a number of years, on a daily basis), would eventually poison the witch, ultimately leading to their death, even if they were witch-vampire hybrids.
=== Divine Being ===
Gods, as well as their offspring, possess divine blood, meaning magic cannot hold them. Specifically in regards to Ben, he was able to cross the threshold where the gods of his village lived as well as pass through Cleo's barrier spell unobstructed. Similarly, Hope was unable to use a locator spell to track Jen after she had stolen the sarcophagus. Despite this, some magic can affect them, as Ben was given an enchanted bangle and had assumed that Cleo healed his wounds, though the latter could not.