Immune system in central nervous system does not work the same way as in periphery. While parenchyma is completely isolated from surrounding tissues, blood-brain barrier, choroid plexus and cerebrospinal fluid are not. They are in constant contact with periphery which makes them perfect candidates for transfer of peripheral immune cells to central nervous system in cases of brain injury or other diseases. Tight junctions present in endothelium of blood-brain barrier and epithelium of choroid plexus make it difficult for peripheral immune cells to cross those barriers, but transfer can still be achieved via adhesion molecule-ligand interaction. Expression of those is upregulated during inflammation. In case of inflammation T and B cells, macrophages, dendritic cells, neutrophils and other immune cells start invading into the parenchyma. Microglia, astrocytes and oligodendrocytes also react to the inflammation. Generally speaking, we want cells to act in anti-inflammatory fashion to protect the neurons, but usually the opposite happens. Working mechanisms of cells that are part of the immune response in central nervous system are complex and specific for different injuries/diseases. To find new diagnostic methods and successful treatments of diseases of central nervous system, we need to see the whole picture of what is going on in the brain and meninges during inflammation.
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