Blood is a specialized bodily fluid that delivers necessary substances such as nutrients and oxygen to the cells and transports metabolic waste products away from those same cells. Vampires, Originals, and hybrids need to drink blood to survive. If a vampire cannot feed on blood for a while they will weaken, to the point of desiccation, the point were they will be paralyzed. Also, blood provides their strength. Most vampires drink human blood, but there are some that choose to drink animal or vampire blood such as Stefan Salvatore and Mikael. However, animal blood does not provide the same level of nourishment to vampires as human blood does.
Types of BloodEdit
Feeding on human blood completes the final phase in transitioning into becoming a vampire; if an individual chooses not to feed, and resists human blood, the transition will not be complete and the individual will die (Like Bill Forbes did in Bringing Out The Dead, with his own daughter by his side). 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 the lifestyle. When a vampire experiences moments of hunger, or bloodlust, or other heightened emotions, their vampirism becomes evident in facial changes- their eyes redden, veins around their eyes darken and protrude, and their fangs become exposed. The majority of vampires drink human blood. Some choose to acquire blood bags to satisfy their blood needs, but others either feed off of willing or compelled individuals, or simply feed and kill their prey. Animal blood does not provide the same level of nourishment to vampires as human blood does. Human blood makes a vampire considerably stronger than those who drink animal blood, keeping their strength and abilities at their peak. When Stefan and Damon returned to Mystic Falls, Damon easily bested his younger brother because he drank human blood, while Stefan stuck to a strictly animal-only diet. It's shown that witch blood seems to have the same effect on vampires as human blood. It is shown that in witch blood can be used to complete a transition into a vampire as a substitute for normal human blood.
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 however and vampires that drink animal blood have decreased strength, decreases speed, decreased healing abilities and a smaller resistance to pain. Since it is stated that to complete a transition into a vampire, one must drink human blood, then presumably, animal blood will not suffice when a human is in transition to a vampire.
Vampire blood is needed to transform a human into a vampire; the human must have vampire blood in their system at their time of death in order to transition. Vampire blood also has strong regenerative properties and when ingested by a human it will heal them of almost any wounds within seconds. Vampire blood does not cause regeneration or causes the person to turn if the person is already dead before the blood is ingested. Vampires typically bleed less in comparison to humans, because of their superhuman strength, healing and endurance. A vampire will bleed similarly to a human, but it requires much more force to damage them, aside from an attack utilizing a vampire's weaknesses. If they are not attacked by means of a weakness, they will heal quickly, if not instantly. Vampires can also feed off of the blood of other Vampires as demonstrated by Mikael. While werewolves don't exactly need to drink vampire blood to survive, it's confirmed that werewolves transformed under the full moon do/can/will eat vampires alive if they catch them. According to Damon and Stefan's reaction afterwards, the sharing of blood between vampires is an intimate and personal experience. It is shown in the episode, 'She's Come Undone', that Caroline injected her mother with her blood via a syringe. This healed her and prevented her from dying; it was a last resort as Liz would not drink from Caroline directly due to the fact that she was unconscious.
In Plan B, it is confirmed that vampires can drink werewolf blood. Before werewolves transform they bleed normally, like a human does. Most likely as a werewolf it is harder to damage them, but how hard it is to damage them is currently unknown.
Doppelgänger blood is needed in the sacrifice ritual to break the Hybrid Curse placed on Klaus. When the doppelgänger was alive, her blood was the key ingredient needed by Klaus to successfully make hybrids, just like Tyler Lockwood, Tony and Mindy. However, after Elena turned into a vampire, her blood couldn't create hybrids. Also, since Rebekah destroyed Klaus' last remaining blood bags of Elena's blood, Klaus can't create any more hybrids. This is why Klaus wants to find the cure for Elena, so that he can continue creating more hybrids . The blood of a doppelgänger is the best option to link several beings or to fortify spells made in supernatural beings. Example: The Curse of the hybrid, the creation of the Originals. Elena's blood was also used by Esther to create an Enhanced Original, Alaric.
Only Klaus' blood can cure a werewolf bite as well as create more Hybrids.