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(sorry, I couldn't resist the rhyming, silly title)

I find it facinating that humans can suffer from an amoeba infestation. Amoeba must reproduce far more slowly than bacteria. In size, they seem to be approximately the same size as the various leukocyte cell types (I see estimates ranging from 10μm to 30μm in diameter). I would assume that they are slow-moving, or, at least no more motile than a leukocyte.

How is it, then, that a population of amoeba can take up residence (and a particularly dangerous one, at that) in a human body without them being wiped out by the immune system? Do we know much about the pathology of such an infection? Do the amoeba actually consume the leukocytes by phagocytosis before they can be consumed themselves?

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How can I add a Greek letter "mu" for "micrometer"? –  Brandon Invergo Jul 9 '13 at 11:26
    
Also, this could probably use some better tags like "protozoa", "amoeba" or "phagocytosis" but I cannot create new tags yet. –  Brandon Invergo Jul 9 '13 at 11:27
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This is nothing I really know or could prove, but wouldn't amoeba be in a region where there is no blood, like the gut? Then the problem is more about surviving in this environment. Additionally I don't think the bacteria are in a way actively "running away" from an immune response. This is not about speed, it is about adaption. –  skymninge Jul 9 '13 at 11:37
    
Hm, that could be but according to Wikipedia, at least, advanced infestations make it into the blood stream though. –  Brandon Invergo Jul 9 '13 at 11:48
    
Also, I didn't mean to imply that bacteria were running away from an immune response but I was positing that amoeba, in theory, could. –  Brandon Invergo Jul 9 '13 at 11:49

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Assuming you don't mean a single leukocyte as I doubt That a lone leukocyte could do much. Macrophages and neutrophils can release their lytic/degrading enzymes spewing them on to the amoeba. Antibodies produced by BCells, particularly the IgE type can coat amoeba and cause histamine release from mast cells recruiting more neutrophils and macrophages. Eosinophils are also important. Finally one must not forget the complement system, a series of proteins that could kill by making pores in the amoeba membrane. Amoeba have however developed many techniques for immune evasion, such as diverting the immune system via cell signalling (cytokines) such as IL10 which dampens the immune system or coating themselves or activating the "wrong" immune system.

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Beat me to the punch. The idea you are looking for/asking about is parasite immune escape/evasion. Here is a good article on the subject. You can find many more, but remember that pathogens evolve with the hosts they infect. I agree that a lone leukocyte wouldn't be able to do much of anything without the rest of the immune system coordinating an interaction. Also a successful infection of a human is unlikely to come from a single amoeba but a large group of amoebas. –  Atl LED Jul 9 '13 at 18:02
    
Thanks for the response. I have found a rich set of literature pointing to the expression of cytokines as an immune response to amoeba, resulting in a strong and potentially damaging inflammatory response. Also, of course I didn't really mean only one amoeba and one leukocyte; that was just for my admittedly lame question/title. –  Brandon Invergo Jul 10 '13 at 9:46
    
I would argue against any specific evolution of amoeba in response to the human immune system, though. Amoeba are not obligate parasites of humans or mammals in general; they do just fine in their natural habitat of small bodies of water. I would argue that upon coming in contact with a human, they just find themselves in a new, cell-rich environment where they can carry on doing their usual thing. Selection for success in their natural, aquatic habitat is almost certainly far stonger than selection for success in mammalian hosts. –  Brandon Invergo Jul 10 '13 at 9:51

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