My magical system has this feature.
The way this is described, in my work, is via souls:
Magical power permeates the natural world, and during gestation, it concentrates in sensate beings. The crystallized magical essence is a soul, and during the lifetime of a person, that soul leaks magical energy. [Magical users] are unique in that their bodies have two specific adaptations which make use of that leakage:
The first is a shield. This shield repels all direct effects. The body of a [magic user] is inviolate from all direct magical alteration by outside forces. It operates as a vestigal, autonomic, form of personal defense, and its energy cannot be used for spellcasting. Or to be more precise, conscious manipulation of magical shielding is somewhere between conscious control of a heartbeat, or breathing, in terms of difficulty.
This shield also inflicts countermagical effects on area-based spells in the immediate vicinity of the [magic user].
Further leakage constitutes the aura which can be used for spellcasting by various means.
Because the crystallized mana of a soul is orders of magnitude higher than what is used for spellcasting (e.g. a soul may contain millions or billions of times the mana used by mid-grade spellcraft), using that leaked mana has no negative effects. Though a [magic user] attempting to use higher amounts of mana than what their aura contains may produce a sort of conversion effect, which violently, and lethally, transitions their magical power from the more dense state (soul) to a less dense state (mana).
It doesn't end well for them.