John Amsterdam is a brilliant NYPD homicide detective who is immortal. He is over 400 years old, but his body structure is that of a 35-year-old.
Born on June 1, 1607 as Johann van der Zee, Amsterdam was a Dutch soldier in the year 1642 when he was stabbed through the heart after he stepped in front of a sword to save the life of a Native American girl during a massacre of her tribe. She and the women shaman of her Lenape village in turn rescued Amsterdam by weaving an ancient spell that conferred immortality upon him. He will not die or age until after he finds his one true love, and only then will he become whole and ready for mortality. His immortality has resulted in Amsterdam spending more than three centuries marked by loss as his friends, lovers, children (63), and dogs (which John numbers - his current dog is named "Thirty Six") gradually grow old and die while he is forced to remain alive. Amsterdam is also a recovering alcoholic who regularly attends Alcoholics Anonymous meetings, having remained sober since 1965. In his lifetime he has joined the Army three times, in addition to the Coast Guard, Marines, and Navy. He stated that he never joined the Air Force because he "doesn't like heights". He has taught history at King's College, served as a physician during the American Civil War, was a furniture maker at the turn of the century, a portrait painter just before the outbreak of World War I, and by 1941 an attorney. At some point he attended Brown University and spent ten years in the CIA.
The phenomenon called "magic" involves accessing and controlling certain energies and entities from other dimensions. Because these other dimensions possess physical laws that are tremendously different from our own, these extradimensional forces defy the universal laws of physics. The "spell" that the Lenape cast on John Amsterdam was the summoning of an extradimensional electromagnetic field that is a sentient energy-based lifeform. The lifeform has bonded to John's body and taken form as an inconspicuous energy pattern linked to the natural electromagnetic field of his body. Since the EM field lifeform permeates John's entire body, it innately possesses an intrinsic understanding of all aspects of his physiology (including DNA) to a degree that is beyond human comprehension. That's why it knows how to manipulate his physiology to grant him immortality.
The EM field lifefom is also connected to John's consciousness and subconsciousness and that's how it knows who will be his "soul mate." It emits a telepathic field that scans the minds of women around him to find the woman who would be the genuine 'perfect match'. When John is in the presence of that match, the EM field lifeform alerts him by giving him a heart attack. When he finally finds that match and they fall in love and the EM field lifeform scans from both John's mind and his match's mind that based on their individual psychological profiles they could never fall out of love with each other...the EM field lifeform will unbind itself from his physiology, thereby restoring his mortality.
Because of the EM field, decapitation is the only thing that could ever permanently kill John Amsterdam. The EM field lifefom repairs his body whenever it's damaged by rebuilding his flesh from scratch by stimulating the cells still there to divide far more rapidly than normal, and revives him from temporary death even after his main organs have stopped operating. It acts like a jump start on cells and John's major organs to get them going again. How long it takes wounds to heal depends mostly on the type of wound and location of the wound.
The EM field speeds up the healing process to such an extent that it happens within minutes, and the process is perfect, leaving no scars or marks because the EM field allows John's cells to access the genetic blueprint to a greater extent than normal human cells. But the EM field lifeform cannot replace parts lost from the external body, meaning his body still cannot regenerate limbs or other appendages. His internal organs do get regenerated. Because they are contained inside his body cavity, the EM field is very concentrated around them and through them. If John's heart, for example, is cut out, he will, of course, die temporarily. Then a new heart will be grown by the EM field lifeform and he will revive.
Another astonishing benefit of the EM field lifeform is that it completely protects John's cells from free radicals and ambient radiation, preventing any oxidative and ionization damage to his DNA. In addition, the EM field has an effect on certain protein structures to drastically stimulate the production of telomerase (an enzyme that prevents chromosomal degeneration), so John continuously possess the enzyme in every cell in his body. This completely prevents cellular senescence and therefore gives all of his cells replicative immortality. Due to a perfect cellular replication and no added damage to DNA, the development of a degenerative condition or cancerous cells in John's body is impossible. The EM field also suppresses the functions of the genetic sequences responsible for the natural metabolic processes involved in aging. As a result of these effects of the EM field, since his first "death" John has become physically incapable of aging.