Differentiating phagocytic microglia can be accomplished by testing for expression of Major histocompatibility complex (MHC) class I and II during wallerian degeneration.[19]The rate of clearance is very slow among microglia in comparison to macrophages. Possible source for variations in clearance rates could include lack of opsonin activity around microglia, and the lack of increased permeability in the blood–brain barrier. The decreased permeability could further hinder macrophage infiltration to the site of injury.[2]