Immortal physiology is different from normal human physiology because of the Quickening, which is basically a very powerful electromagnetic field. Immortals look and act on wounds, diseases exactly the same way as ordinary humans do until they die for the first time. After that the only thing that can kill them, permanently that is, is decapitation.
The Quickening rebuilds their bodies whenever it's damaged by rebuilding their flesh from scratch by stimulating the cells still there to divide far more rapidly than normal, and revives them from temporary death even 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, but also on the type of wound and location of the wound.
The Quickening 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 Quickening allows Immortal cells to access the genetic blueprint to a greater extent than mortal cells. The old "wounds" of an Immortal aren't healed, because they're not wounds. They're already healed, and so nothing will be done about them. If they're reopened they'll heal normally, unless some of the scar tissue is still present. But the Quickening cannot replace parts lost from the external body, meaning the Immortal body still cannot regenerate limbs or other appendages.
The internal organs do get regenerated. Because they are contained inside the body cavity, the Quickening is very concentrated around them and through them. If an Immortal's heart, for example, is cut out, the Immortal will, of course, die temporarily. Then he or she will revive. A new heart will be grown by the Quickening.
Cuts and scratches on the body heal very quickly, but on the neck and head it takes longer. Deep cuts on the neck are likely to leave scars, and are slow to heal. Also the organs inside, because when the neck is cut open, slashed or gnawed or ripped, the organs inside are no longer completely surrounded by the flesh, thus impeding the process of the Quickening. This is because the Quickening is least concentrated in that part of the body. It is the area where all the neurotransmitting takes place, very near the medulla. Logically, the Quickening is less concentrated in these areas so that it doesn't interfere with the functions of the brain and spinal cord.
A minority of Immortals are actually capable of breathing under water. This is because their electric component just splits up the water into hydrogen and oxygen, they then breathe the oxygen.
Clarifications. A wound to the neck would be treated as any other wound. The example in the first Highlander film shows the Kurgan being sliced across the neck by Ramirez and then having the mark hundreds of years later. Immortals can empower their swords with the Quickening, as shown by the blades sparking when they are connected or struck together. This empowerment is what caused the permanent wound. The Quickening disrupted the Kurgan's ability to heal the wound. This empowerment has been shown dozens of times in the film and the series.
That electrical 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, attract metal objects to their bodies, 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 is tuned into the frequency of Immortal's Quickenings perfectly; it acts like a warning beacon. Immortals sense one another when they come within a certain range of each other, in much the same way two magnetized objects will respond to each other's presence when brought close enough together. It’s usually described as vibrations felt in the skull. That's because the head is the most sensitive part of the body. The pull of the Quickening is also felt elsewhere.
Another astonishing benefit of the Quickening is that it completely protects cells from free radicals and ambient radiation, preventing any oxidative and ionization damage to the DNA of an Immortal. In addition, the Quickening 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. Due to a perfect cellular replication and no added damage to DNA, the development of cancerous cells is impossible. The Quickening 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 legendary "Prize". According to legend they must fight each other until there's only one Immortal left. When there are only a few Immortals left, they will come together in a "Gathering", where they will fight till the last. The last one gets the Prize. Nobody knows what it is exactly, but it's supposed to be so powerful someone can rule the world with it. And that's the whole point; even if an Immortal doesn't want to fight, or believe in the Prize, other Immortals will come for their head, because they do and they have to know how to defend themselves, or they die.
Because the limbs and appendages 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'll gain his experience, his power, and his skills, given they train themselves in them, and that of all the Immortals' Quickenings he took, and they took, etc. The more power you possess, the quicker you heal and the easier you survive. If someone would become the last, than he would have all the combined power of every Immortal who ever lived. That's power and there are a lot of Immortals hunting for heads in order to gain that power. It in itself is a big enough Prize for a lot of Immortals to kill.
When an Immortal is shot, the bullets are absorbed by the Quickening. When an Immortal is stabbed or skewered, of course the weapon or tool is not absorbed. Earrings and nose pins are not absorbed. This is because the Quickening must be surrounding the object, and able to flow through it. When a solid object is completely contained inside the flesh of the body, the Quickening dissolves it. If something is swallowed, it will not be dissolved. It will simply pass through the digestive system like anything else that is eaten. If something is not completely inside the body, for example, if a knife is stabbed into an Immortal's heart, it will not be able to be dissolved because part of it is still outside, therefore the flesh is unable to be completely around the object.
Clarification Bullets would not be absorbed. The bullets would be forced out of the body. The body rejects damaging foreign matter.
Pre-Immortals already have an elevated energy field. They feel a little different than mortals, but their "buzz" is not as powerful as that of Immortals. This additional energy comes from their mothers. As a result, up until recently mothers of Immortals died at birth because of the energy depletion as they give their energy to their Immortal children.
Immortality is not genetic; Immortals are being born all over the world, in different circumstances, different circles, and different environments. It is instead transferred through energy. Every cell carries the entire DNA string. Cells and DNA strings are electromagnetic factories: each cell generates an EM field and the DNA inside a cell influences the exact make-up of that field. The EM fields of cells can essentially carry the information of the exact DNA inside it's core. Thus when EM fields from different animals interacted, when they came close to each other, the information of the DNA, and thus the mutation, could possibly be transferred through electromagnetic induction.
When a woman comes in contact with an Immortal, his Quickening, which is basically the EM field generated by their cells, if it is powerful enough and has the correct frequency, can transfer the information in the EM field required so that when she becomes pregnant, the EM field is given to the child, who will be born Immortal. After all, since the Quickening can simply stimulate cells to divide incredibly fast, rebuild any damage to their bodies, and absorb energy - which is not limited to electricity, it can certainly pass on the Immortality traits through the EM field.
For thousands of years, mothers of Immortals would die at birth because of the energy depletion as they gave their energy to their Immortal child. In the old times, children whose mothers died at birth were raised, of course, although many would look at the child distrustful. Some would even consider them 'children of the devil'. Immortal children of course, get their energy via the umbilical cord, which would become like a high voltage line, which since the Bronze Age would be cut with metal scissors. Which would result in a small electric discharge, sealing their fate as children of the devil. The children were simply thrown away somewhere or left at an orphanage or church as foundlings.
The way the energy is transferred would limit, especially in the old days, Immortal children to lower classes. The higher classes usually didn't socialize with lower classes, resulting in less contact with an Immortal's EM field, or someone who carries the information of that field inside of him or her, thus they would not be likely to get an Immortal child. Lower classes had higher frequencies of single mothers; prostitutes, widows, victims of rapes or simply a woman without a husband, all mothers with a higher chance of coming in contact with an Immortal's EM field and when they die in childbirth the child would again be a parentless child, a foundling.
Everything increases the chance of foundling Immortals and limits the chance for an Immortal raised by his or her biological parents. From now on, with today's lack of superstition and increased quality of medical care, there will probably be a lot more non-foundling Immortals.
Sterility is one of the main characteristics of Immortals. When female Immortals are pre-Immortal, they menstruate just like other women. After they become Immortal, however, they don't. This is because of the spontaneous healing qualities of the Quickening. The blood and tissue lining of the womb is unable to form, because the tiny blood vessels that supply it are unable to rise to the surface and open up. Also, ovulation is impossible, because the ovaries are also affected by the Quickening. A pre-Immortal woman can't have children because the Quickening is so strong and concentrated, even before the onset of pure Immortality, the ovaries are unable to release the ova.
Male Immortals are sterile because the concentration of the Quickening interferes with the mitochondria in the sperm cells. The mitochondria are the things that give the sperm energy. The Quickening increases their energy to such a degree that it destroys the enzyme on the head of the sperm, making them unable to penetrate an egg cell to fertilize it.
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 hand to hand weapons. Rule number four, the most important one: There can be only one.
Those are the rules they live by. Not even the most despicable of Immortals, will break the no fighting on holy ground rule. No fighting on holy ground is not much more than a convention these days, but it was necessary, and still is really, for survival earlier.
There exist something called "earth radiation." It's a name encompassing any number of phenomena: the Earth's natural magnetic field, radiation coming from any mineral in the ground, electro-gravitic, multi-dimensional energy transfer, and a whole lot more.
Crossing magnetic field lines, radiation and magnetic fields from minerals in the ground, these focal points have very powerful effects. The mostly documented are the negative effects. The frequency and polarity of them disrupt a person's natural EM field. Results are: people are more aggressive, get sick more often, get depressed more often, suffer migraines, stay sick longer, wounds take longer to close, and so on. These bad points have the same effects on Immortals. Of course their own field is so powerful they barely notice a thing, but it also weakens a Quickening.
The opposite isn't that well known, mostly because nobody complains about being extra extraordinarily healthy. They were also given a special name: Ley radiation and Ley focal points. The special name is misleading, in essence, they are the same as the focal points with negative effects. The frequency and polarity of Ley beams and points, however, doesn't disrupt, but rather strengthens a person's natural EM field. They're happier, their wounds heal faster, they get sick less often, and other beneficial effects. Centuries ago, before a Pope had all original references to these natural points of energy removed, they were called Holy Ground.
They used dowsing rods to find these points and built churches and other religious and important buildings on it. Over time, the Holy Ground from the rule - the Ley points - became one with, and finally changed to, all religious holy ground. Just as negative radiation, weakens the effect of a Quickening, Ley radiation strengthens the effect of a Quickening. This influx of energy would incinerate an Immortal.