Once microplastics fall apart futher, to nano-plastic, it will start to get absorbed by T cells because they want to destroy any invaders. Once absorbed, T-Cell start to produce H2O2 to destroy anything they absorbed. Unfortunately, plastics are mostly chemically neutral and so, it cannot be destroyed like that. T-Cells produce more H2O2, eventually it leaks outside and start inflamation of surrunding tissue. There is research about it.

There’s a transition point where things stop being micro plastics, then nano plastics, and become specific chemicals.

Those molecules may be toxic but the interactions are distinct from microplastics or nano plastics.

Unknown to me, but something useful to know is that there is something smaller than microplastics called nanoplastics. The distinguishing factor is that nanoplastics are particles smaller than 1 micron, while microplastics are particles between 1 micron and around 5 millimeters. As your other respondent notes, at some point you're talking about single molecules. As plastics is an entire category and not a single thing, there's no one size where that happens, but some polymers have chains that are as little as 0.01 (1/100th of a) micron in size.

As far as I am aware, we have yet to have effective, replicable research on what if any biointeractions exist with nanoplastic particles, including single polymer chains.