Microglia is normal macrophages in the brain much the same way as Kupffer cells in the liver and Langerhans cells in the skin. These organ-specific macrophages are specialized in the development period. In contrast, F4/80+ infiltrating macrophages are activated by inflammation. For instance, cirrhosis (liver fibrosis accompanied by HBV/HCV infection and NASH) is rich in F4/80+ macrophages, however it is quite difficult to distinguish the proliferation of originally existed Kupffer cells from the infiltration of macrophages from the bloodstream by the inflammatory cytokines such as IL1-beta and TNF-alpha etc. After all, both populations are positive for F4/80. This kind of phenomenon can be occurred also in the brain edema due to increased extracellular glutamate in the GBM. 

Microglia is normal macrophages in the brain much the same way as Kupffer cells in the liver and Langerhans cells in the skin. These organ-specific macrophages are specialized in the development period. In contrast, F4/80+ infiltrating macrophages are activated by inflammation. For instance, cirrhosis (liver fibrosis accompanied by HBV/HCV infection and NASH) is rich in F4/80+ macrophages, however it is quite difficult to distinguish the proliferation of originally existed Kupffer cells from the infiltration of macrophages from the bloodstream by the inflammatory cytokines such as IL1-beta and TNF-alpha etc. After all, both populations are positive for F4/80. This kind of phenomenon can be occurred also in the brain edema due to increased extracellular glutamate in the GBM.

Macrophages have been shown to be CD11b+/CD45high, whereas microglia are CD11b+/CD45low. Moreover, CCR2 has been used to distinguish macrophages from microglia. Recent publications indicate that CD169 might be a suitable marker to specifically label microglia.

thanks Bjorn and Yoshida