Note: What follows is largely speculation and has very little to do with the canon as established by the movie, and the TV series.
Immortal physiology is different from normal human physiology because of the Quickening, which might be a very powerful electromagnetic field. Immortals suffer and die from wounds exactly the same way as ordinary humans do, however, they restore to a healthy state after dying. The only way to permanently kill them is decapitation.
Immortals superficially are the same as mortals in every sense. They seem to share the exact physiological processes, and are not markedly stronger, more intelligent, nor more able than mortals. They also differ in appearance, strength, intellect and size, as do mortals, including extremes, such as the simple Mikey.
Immortal anatomy and physiology is identical to that of mortals. Immortals must eat and sleep; they can become intoxicated or drugged. His or her Quickening cannot be detected via any known medical or technological apparatus. It's possible that the Quickening is a form of neural or bio-energy field far beyond the capability of late 20th century or early 21st century technology to detect.
Similarly, many mortals, upon meeting and interacting with an Immortal, see no evident difference between them in form, appearance or substance from themselves. Tessa Noel, Duncan's long-time girlfriend, was shocked to learn of her lover's Immortality at first, though in the long run, it did not hinder their relationship.
It could be said that Immortals are a sub-species of humanity, due to being very similar to mortals despite their Immortality.
Immortals in all films/series can be found in any era and in any place around the world. They can be of any race, ethnicity, or gender. For example, Xavier St. Cloud is a Moroccan, Carl Robinson and Richie Ryan are American, Luther and Haresh Clay are West Africans, May Ling Shen, Kiem Sun and Jin Ke are Chinese, Kanwulf is a Viking, Axel Whittaker is Swedish, Sean Burns, Annie Devlin and Tommy Sullivan are Irish, Reagan Cole is Swiss, Hamza el Kahir was a Bedouin, Rebecca Horne was a Mycenaen, and Karros was a Thracian, to mention just a few.
The Quickening rebuilds their bodies whenever it's damaged, and revives them from temporary death after the main organs have stopped operating. It acts like a jump start on cells and the major organs to get them going again. How long it takes wounds to heal depends mostly on the individual, and on the strength of their inherent Quickening, but also on the type of wound and location of the wound and the power level of the Immortal in question.The Quickening speeds up the healing process to such an extent that it happens within seconds with small wounds, and the process is perfect, leaving no scars or marks. Scars and other damage from before the first death remain, however, because they are no longer wounds; they are already healed, and so nothing will be done about them. The Quickening cannot seem to replace limbs; once an appendage is removed from the field/body, it is apparently no longer subject to the power of the Quickening. Cartilage and all soft tissue except that of the throat from a cut that nearly beheads the Immortal will restore to pre-injury state.Cuts and scratches on the body heal very quickly, but on the neck it takes longer. Deep cuts on the neck are likely to leave scars, and are extremely slow to heal.
The internal organs do regenerate. If an Immortal's heart, for example, is destroyed, the Immortal will, of course, die temporarily. Then he or she will revive. A new heart will be built by the Quickening as the heart and other organs remain within the field/body affected by the Quickening.
Some Immortals are apparently actually capable of breathing under water. When they leave the water, their body will restore to normalcy.
First Death Edit
Immortality is triggered through a violent or unnatural death.
The process following First Death is not fully known, but it's possible that the Quickening "recognizes" that the host body has died, and thus regenerates the individual with the full Immortal state. It could be a function of the brain that kicks in once a pre-Immortal dies and becomes fully Immortal.
That energy that is the Quickening is so well integrated into the day to day workings of Immortals' metabolisms as to be completely unnoticeable; they don't give off sparks when they shake hands with people, disrupt the workings of radios when they walk by, or cause any other overt electromagnetic phenomena during their normal functions. However, they do have a sense that acts upon EM-fields with induction. It's tuned into the frequency of Immortal's Quickening perfectly; it acts like a warning signal. Immortals sense another of their kind when they come within a certain range of each other, in much the same way two magnetized objects will respond to each others presence when brought close enough together. It is sometimes described as a headache (The Immortal Cimoli).
Another benefit of the Quickening is that it completely protects cells from free radicals and ambient radiation, preventing any oxidation and ionization damage to the DNA. In addition, the Quickening likely modifies and augments the genes responsible for telomerase (an enzyme that prevents chromosomal degeneration) production so Immortals continuously possess the enzyme in every cell in the body, completely preventing cellular senescence and therefore giving every cell replicative immortality. The Quickening probably also effects the genes responsible for the metabolic processes of aging, by completely suppressing their function. As a result of these effects of the Quickening, Immortals are physically incapable of aging after their first "death".
There's more to Immortality than healing and not dying. Immortals duel each other to the death (via decapitation) for a "Prize". According to legend, they must fight each other until there is only one Immortal left. When there are only a few Immortals remaining, they will come together in a "Gathering", where they will fight till the last. The last man standing gets the Prize. Nobody knows what it is exactly, but it's supposed to be so powerful as to enable the winner to rule the world with it.
Because the limbs and appendages apparently cannot regenerate, an Immortal is truly dead beyond any sort of revival when their head is cut off. When an Immortal's head is cut off, his Quickening, his essence, his energy matrix, his power, is transferred to the closest Immortal, given he or she is within a certain radius. They will then gain his experience, his power, and his skills. The more power one possesses, the quicker one heals and the easier it becomes to survive.The winner of the Prize would have all the combined power of every Immortal who ever lived.
Pre-Immortals apparently already have an elevated energy field. They feel a little different than mortals to Immortals, but their "buzz" is not as powerful as that of Immortals. Some Immortals can recognize a pre-Immortal upon meeting.
The source of their Immortality is unknown; Immortals appear all over the world, in different circumstances, different cultures, and different environments. "From the dawn of time we came, moving silently down through the centuries. Living many secret lives, struggling to reach the time of the Gathering, when the few who remain will battle to the last. No one has ever known we were among you...."
Sterility is one of the main drawbacks of Immortality. No Immortal is capable of becoming a biological parent, although many have adopted children over the centuries, both mortal and Immortal, to fill this hole.
Immortals live by a set of rules. Rule number one: they do not fight on holy ground - any holy ground. Rule number two: they fight only one on one. Rule number three: they fight only with melee weapons. Not even the most despicable of Immortals, will break the first rule.
The only way to kill an Immortal permanently is by beheading. Death can occur through decapitation by any means, such as combat, an accidental decapitation (Alexei Voshin), or suicide (Timothy of Gilliam and Mikey Bellow).
When an Immortal is beheaded, his or her Quickening is released and attracted to the Quickening energy of the closest Immortal, often the Immortal who killed the fallen opponent. In essence, the Quickening is released upon beheading. Quickenings do not seem to happen if there is not another Immortal close by.
It is possible that an Immortal's Quickening incorporates itself into the brain of the living/victorious Immortal, thus affecting his or her personality and reactions. This may explain the existence of Light/Dark Quickenings. Darius, who experienced a Light Quickening, according to legend, killed a Holy Man who was the oldest Immortal known at that time. He was overwhelmed and altered from warlord to holy man.
Jim Coltec had once taken Duncan MacLeod's darkness into himself to help him, fulfilling his role as a hyoka or holy man, with a stated life purpose of ridding his tribe and the world of evil. As a result, he took the Quickenings of predominantly or perhaps exclusively "evil" Immortals. Over many centuries, Coltec dealt with Immortal evil in the same manner he dealt with human evil, leaving him vulnerable to being overwhelmed after he took the head of Horvan Kant, and falling prey to a Dark Quickening.