Would such a parasitic reaction exist outside of nature itself?

As form (life) within flow (inception towards death) all within flow represent choice based reaction to balance; with balance representing the momentum of flow; allowing the temporary sustenance of form within ongoing flow.

In short...no existence outside flow/form (nature) aka to exist represents to react as form to flow.

EXIST', verb intransitive [Latin existo; ex and sisto, or more directly from Gr. to set; Latin sto, to stand. The primary sense is to set, whence the sense of permanence, continuance.] aka form set into the momentum of flow.

If not

Nature (flow/form aka balance/choice) represents everything offered to the perception of all within; while no; not; nothing; nothingness represents the choice to ignore everything for the suggestions of others. In other words...nature (flow/form) offers inspiration through movement towards choice of reaction; while the suggestions of others deceive one to choose to ignore reacting to balance within movement for consent to believe or not believe the affixed (true or false) information suggested by the choice of others.

In short...balance defines choice; choice of reaction to balance represents "need"; while choice to ignore reaction to balance represents "want"; which tempts to instead consent to the suggested choices of others.

what natural purpose does such a mechanism serve

The temptation of want is needed to sustain the balance (need/want) for choice to react to. As form within flow...form "needs" to sustain itself by choice based reaction to flow (adaptation); yet the ongoing flow constantly tempts the temporary form to "want" to ignore the needed struggle for sustenance.

Flow represents ongoing velocity; form within the momentum of flow represents temporary resistance to velocity. Both are needed for balance. Velocity represents loss of potentiality; resistance represents growth of potential.

In short...need demands adaptation to inspiration; want tempts to ignore it. Can't have one within out the other; because balance defines choice having two choices (need/want).

is there a natural defense against such a parasitic reaction?

One needs to resist the temptation to choose want over need; while struggling to choose need over want.

Nature offers inspiration; we need to adapt to it by choice of reaction; while resisting the temptation to consent to the suggested information by the choice of others.

Lastly...utilization of implication (if/then) aka in accordance to flow/form; while resisting utilization of reason (true vs false) aka a conflict caused by consent to suggestion; allows one to defend oneself from the parasitic suggestions of other ones.

Natural law (flow upon form) operates through offer/consent aka flow offers balance; form consents by choice of reaction within it. The parasitic few exploit our ignorance of natural law; to invert balance/choice towards choice/choice in ignorance of balance. Choice responds to need/want (balance); not to suggested choices of others.

To believe or not believe what another suggests; gives the other the power to define it for those who consent by either believing or not believing it. This is how the many find themselves in a conflict of reason about pro-life versus pro-choice without comprehending that a) the suggestion of abortion caused the conflict and b) that life equals choice aka the few taking the piss out of the many by branding the conflict as pro-life versus pro-choice aka a sleight of hand about ignorance for those with eyes to see. It is that ongoing reasoning about suggested meaning that sustains the suggested abortion for over half a century and counting.

Neither side can win reason; because only those who suggest what both sides are reasoning about; hold the power to choose what it means; which is what they exploit by suggesting constantly contradictions to both sides within reason. This is btw called talmudic reasoning.