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.