Major histocompatibility complex
Oligodendrocytes fail to recruit macrophages for debris removal. Macrophage entry in general into CNS site of injury is very slow. In contrast to PNS, Microglia play a vital role in CNS wallerian degeneration. However, their recruitment is slower in comparison to macrophage recruitment in PNS by approximately 3 days. Further, microglia might be activated but hypertrophy, and fail to transform into fully phagocytic cells. Those microglia that do transform, clear out the debris effectively. Differentiating phagocytic microglia can be accomplished by testing for expression of Major histocompatibility complex (MHC) class I and II during wallerian degeneration.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.