Recently thinking about the mechanisms involved in each process it seems that although phenotypically opposite in terms of inducing an immune response, the functional mechainsm, ie. constant low level antigen presentation, of inducing and maintaining tolerance is not so far different from maintaining immune memory in mammals.
You have actually pin pointed the truth, although opposite in action, it all boils down to the same mechanism.
As far as I can recollect, immune tolerance is achieved by things like clonal anergy, i.e. unreponsiveness, etc., whereas immune memory is a part of your secondary immune responses.
What you seem to be talking about is the way in which adaptive immune cells perceive antigen. It is the same whether the antigen is self or non-self, tolerizing or immunogenic. In that sense, the two mechanisms are the same.
What makes these processes different is the context in which the antigen is presented. If innate "danger" signals are triggered, antigen presenting cells display the antigen in the context of activating co-stimulation. In the absence of this innate response antigen is perceived as self and is tolerized.
thanks for your responses... so it is the costimulatory effect during antigen presentation by dendritics cells that is the key discriminator between the two processes? I should elaborate why this question came up. I was just getting familiar with recent discussion about hybrid dendritic cell formation in the transplant context to from "cross-dressed" DCs and wondering how this affects tolerance or immune responsiveness and graft vs host or host vs graft effects. Quite new to the topic so wanted to get better informed insight. It utterly confuses me. thanks.
.. sorry a little extra ramble... I guess I should also add that it seems to me that MHC-transfer likely occurs at some background level as a means of providing constant low level exposure to self antigens and perhaps normalizing antigen distribution on APCs (antigen presenting cells) across that compartment - I could be completely wrong on this but surely it goes undetected if it occurs because it's not something that we would be looking for and wouldn't jump out at us. But it does seem unlikely that such a process is unique to the transplant setting. It is just revealed more clearly to us in this setting because we have some degree of HLA mismatch that can indicate hybrids as has been shown by Lechler and others (http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=PubMed&dopt=Citation&list_uids=16829787).
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