The Triple B: Blood-Brain Barrier
The Blood Brain Barrier is what separates the Brain into its seperate "compartment". The BBB prevents larger substances such as Bacteria from entering the brain whilst allowing smaller essential molecules through like O2, CO2 and Glucose through. This is due to tight junctions between the cells which bind the two cell membrames together and preventing molecules travelling between the cells.
This makes infection in the brain far less likely although it makes infection in the brain more serious and difficult to treat. Since Antibodies cannot pass the BBB and only certain anibodies can pass through (only after the brain having swelled slightly, widening the gaps in the BBB). It was discovered after Ehrlin found, whilst dying animals bodies for microbilogical studies, that the brain cells would not be properly dyed when the dye was injected intravenously. He put it down to the fact brain cells did not take up the dye as well as other cells. However it also happened vice versa, with dye injected into the brain not moving into the body. After the invention of the SEM it was confirmed to exist.
Multiple Sclerosis can be attributed to the fact that a degrading in the lining of the BBB allows T lymphocytes into the brain which are the cells responsible for the auto-immune degradation of the myelin. Anti- oxidants such as Lipoic acid can be used to slow the degradation of the BBB.