system
Given the following post, respond as if you're commenting on Reddit:
human
[The Thing] Why didn't drawing blood for the hot needle test cause the Thing to try and defend itself?

We hear MacReady say that any part of the Thing will defend itself from harm, so why doesn't assimilated Palmer react violently to the scalpel drawing blood?
gpt
In a way, The Thing in question *did* defend itself from harm.

The Thing's goal is assimilation. it accomplishes this goal by stealth and subterfuge. To whatever degree it is sentient, it will choose the most prudent course at any given time to either overwhelm with brute force, or hide and wait for a more opportune moment. 

In that particular moment, reacting violently to the scalpel cut would have done nothing more than alert the humans that Palmer was indeed a Thing. The Palmer/Thing was restrained and relatively helpless at the time, so keeping up the ruse was  the most pertinent course of action, as at that time nobody (likely not even the Thing) knew whether or not the blood test would actually work.

As to why the Thing contained in the blood tray reacted the way it did, rather than remain inert and continue the ruse: if we can safely assume that every particle of the Thing is somehow self-aware to a degree (which Mac proved via the blood test), we have a somewhat lesser understanding of how aware/connected each part of the Thing is of the current status of it's *other* parts that are not currently connected to the specific piece of biomass in question.

The Thing contained in the blood sample that touched the hot needle reflexively tried to defend itself by invoking it's flight/fight response and jumping away. It is likely that there was simply too little biomass in the tray to make a wholly-sentient Thing capable of logical thought. Enough sentience to react, but not enough to think about how it was reacting. This goes some way towards proving that the "overall" consciousness of the Thing resides in each individual piece of mass. I.E. the "parent" Thing likely cannot directly control parts of it's own biomass that have been separated from the whole. However, the parts do seem to have a sort of Corsican brothers-type sympathy and they all share a common goal, but they are effectively individual actors when separated.