Harry Potter: Unexpected Immortal
The production of the Elixir of Life is similar to a ritual.
The process is as follows:
The stone should be soaked in blood.
Basilisk venom and phoenix tears are to be added.
Essence of dementor is added to the mixture.
Physical part of a dragon and the venom of acromantula.
These are the ingredients required.
The final and the most important step is subjecting the mixture to pure magic i.e. focused magic not in the form of a spell.
Harry James Potter has always been adaptable. As such his magic is too. A significant part of his magic was dedicated to fight the Horcrux within and to restrict it from influencing him.
At the end of the final task when Voldemort took his blood, he did not take the blood protection but the horcrux within Harry escaped back to its owner as it sensed its imminent destruction by Harry's magic. With the escape of the horcrux, harry's magic was free of its confines. Another event occurred during this was the collapse of the blood wards surrounding #4, Privet Drive.
1st year: Shard of Sorcerer's/Philosopher's Stone in Bone Marrow.
2nd year: Basilisk Venom and Phoenix Tears in Blood Stream.
3rd year: Dementor Essence during train ride and Sirius' rescue.
4th year: Nail of Hungarian Horntail in Bone Marrow during the First Task. Acromantula Bite during the Final Task.
Each and every year during these events his magic adapted itself to better suit him.
Once his magic is free, it focused on the different items present within Harry. As his magic is always adaptable, with the innate magic still present in the ingredients within his blood, the Elixir of Life is created within Harry's Blood. Thereby making him essentially an Immortal.