In an imaginary sense, If you alone were replicated into 2 so that there should now be 2 of "you" (meaning you both have the same DNA). Then let's say one of you shrunk to the size of a bacteria, and was placed inside the other you (normal human size. Let's call this the actual you).
The question is how would your immune system affect the bacterial sized you in your own body?
Would white blood/T cells try to get rid of the smaller you? If so, how? And could the smaller you die from this?
This is a very interesting question. As others have alluded to, the body generally recognises self and non-self. However, as with any biology it's not that simple. Even leaving aside autoimmune disorders, the body doesn't always attack non-self and doesn't always leave self alone. Think of a pregnancy - the foetus is non-self, and yet the body doesn't reject it. The same can be said for a whole host of molecules that enter the body, such as food or things breathed in (e.g. pollen). For this reason, Charles Janeway put forward the infectious non-self theory. This postulated that the body had a degree of leniency towards non-self, and would only attack if the macrophages became activated. This is done through recognition of Pathogen Associated Molecular Patterns, or PAMPs, binding to Pattern Recognition Receptors, PRRs. These PRRs are receptors present on immune cell surfaces, though most relevantly on macrophage surfaces, and when they bind to PAMPs it sends a message to the cell that it's likely in contact with a pathogen (or at least there is likely a pathogen in the area somewhere). This causes activation, and starts an immune response.
However, it's also too simplistic to suggest that the immune system always leaves self alone, and in fact one of the key jobs of macrophages is to clean up self cells that are damaged, and undergoing apoptosis - programmed cell death. (You can read more on macrophages here and more on apoptosis/other methods of cell death here). Therefore Polly Matzinger put forward what is known as the "Danger Model". This basically speculates that in order for an immune response to occur, the body has to recognise certian molecules that are evolutionarily associated with danger. PAMPs would indeed count, but there could also be endogenous molecules such as HSPs - proteins produced when a cell is under duress through heat, pH, cold, irradiation, viral infection etc. This tells the immune cell that detects it that the body is in danger, and allows activation and mobilisation of an immune response.
So the question here is, would the body respond to a miniature version of yourself? That would depend on a number of factors:
Where in the body would you go? If you were to stay in the digestive tract, so get swallowed and 'passed through' the body, I would say it's very unlikely you would be attacked. The body has a high tolerance for places such as the gut, because it's constantly exposed to external and non-self molecules. If it were in the blood stream, it's more likely because you're more likely to come across immune cells.
How is this 'shrinkage' achieved? Growth comes in two ways, through increase in cell size and increase in cell number. Which of these are you reducing? Are the cells the same size, but there are many fewer of them (unlikely, especially given that the opening question specifies to bacterium size, which are many times smaller than human cells already) or do you have the same number of cells but each cell is shrunk to a much smaller size (given that the entire body is the size of a bacterium, each cell would have to be many times smaller than a bacterium). If the cells were the same size, or even a bacterium-sized each, then an immune response is much more likely. If the whole organism is the size of a bacterium, the cells are going to be far too small to elicit an immune response.
Are you causing danger signals? As mentioned above, if no danger signals are present then it's unlikely an immune response will be launched regardless of anything else. Provided you're not killing or injuring cells, the body will pretty much ignore you.
I would say you wouldn't be attacked if you shrunk down as far as you've suggested. Everything would be too small for the body to recognise. However, if you didn't shrink down nearly as far, so your cells remained a size the body could recognise, and then started blundering about causing all manner of danger signals around you, the body may pick up some of your antigen and launch a response against you. On the other hand, the body is likely to have become tolerant to your antigens so even if this was the case, a full immune response is unlikely.
The immune system recognizes patterns - incase of innate immunity and shapes - incase of active immunity.
I am making a few assumptions:
The Shrinking would result in a smaller version of "the person" - meaning he/she is a bacteria sized person capable of executing all actions that a human can do.
The shrunk person is placed inside the blood vessel or some place where the immune cells will have easy access.
Now, the answer would basically depend on two factors:
The pattern recognition: It is higly unlikely that the innate immunity would recognize anything that is so small - As the person himself is the size of bacteria, the pattern recognition would be unable to find the patterns.
The Shape recognition - This might be able to identify a specific structure. The structure in question may be a whole cell or a cluster of cells or a hair follicle, etc... The epitopes on foreign organisms which are recognized by the active immunity are much smaller than the organism itself. Infact a single bacteria may have thousands of epitopes. The epitopes are usually proteins and what is recognized may be as small as 3 to 4 amino acid cluster. Thus this system has the potential to identify and eliminate the shrunk person.
The shrunk person will not be considered as "self" because on shrinking, the "self-shapes" don't apply anymore. Think of it as a glove specifically made for the hand of an adult. A child's hand will not fit into it. Likewise any change in size of proteins will eliminate all question of self and everything in that "new size" is considered foreign.
When your body develops, immune system learns which tissues are foreign and which are innate. So usually immune system does not attack host's cells. Opposite case is autoimmune diseases. Immune system does that by addressing surface proteins of cells. Only foreign cells, never presented before, cause response.
If you think that large-you immune system will not attack large-you cells, then you should agree that large-you immune system will not attack small-you. Except, maybe, for bacteria that are on the skin on small-you and now sitting in blood stream of large-you.
The question boils down to: why my cells are not attacked by my immune system? Answer: because in healthy animals it discriminates host vs invaders due to memory of immune system.
How does the body know you is you?
There's two forms of immunity: innate and adaptive.
Innate
The innate immune system is the first line of defence that basically works on pattern recognition. "If I see these chemicals, they're usually bacteria/fungi/viruses, so I'm going to kill anything with these chemicals." Little you wouldn't have any of these chemicals unless of course the fact is our body is covered in bacteria which might be recognised (were the chemicals big enough to be recognised). Then they also try to kill things when they're worried somethings causing damage. So if they see that the area is inflamed because little you accidentally broke something, they might come and gobble you up because they're extra activated by the inflammation.
Adaptive
As for the adaptive immune system, this looks for certain markers and can identify specific bacteria and things that aren't us. There's these proteins on cells called MHC (a.k.a HLA). Our body cells are covered in OUR ones and everyone has different ones (unless you're an identical twin). So in little you's case as long as those proteins were there, the body would think you're okay.
Its impossible to happend, why worried about it? Maybe you`re writing a fiction book. I cant answered using biological arguments because this is fiction.
