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I have read that neurons have proportionately less MHC molecules than other cells of the body. What is the advantage of this?

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Antigen presentation by MHC will induce a cytotoxic response by the immune system, which is usually a good thing in the body since most cells can just divide and replicate again. Neurons, however, are particularly ineffective at regenerating from such an attack, and are not easy to come by; they are also rather important! Better not to risk it, eh?

That being said, neuronal expression of MHC is actually a pretty complex case, and this open-access article is a good start down the rabbit hole (see also here, here, and here if you have access).

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What happens if a neuron is infected by bacterium, let's say which is definitely gonna kill it ? If it presents an antigen then also it will be lysed and if it doesn't then the bacteria will kill it. –  biogirl Sep 14 '13 at 16:54
    
Are the few MHC molecules there just so that the immune system is warned of the presence of bacteria ? Does that make any difference if few MHC presents the antigen or thousands present it ? Is there a threshold over which a cytotoxic response takes place ? –  biogirl Sep 14 '13 at 16:58
    
@biogirl Listeria monocytogenes does just that and can cause bacterial meningitis, which is highly lethal. Neurons can respond to interferons and can upregulate MHC expression during infection; there's even some evidence that impairment of neuron function itself spurs that process (here and here). T-cells can respond if presented with as few as 3 MHC-peptide complexes. –  Amory Sep 17 '13 at 5:24
    
The brain also has resident immune cells, in particular microglia. –  Amory Sep 17 '13 at 5:26
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Just because they can doesn't mean doing so is particularly efficient. Besides, T-cells aren't floating around through the brain like elsewhere. Usually, it seems, MHC is not readily expressed unless induced by infection. –  Amory Sep 17 '13 at 17:32
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