While limiting the expression of MHC class II to so-called "professional" APCs partially ameliorates the problem of self non self recognition among CD4+ T cells, it remains difficult to understand how it is possible that even these cells' class II molecules are not deluged with processed "self" molecules (for example, albumin is present at extremely high quantities in the serum, so one might imagine that peptides from this alone would suffice to compete for any transiently present non-self proteins). What is/are the mechanisms that would afford enhanced processing or recognition of potentially dangerous molecules?
There are several components of the immune response that together optimize the ability of antigen presenting cells to present foreign peptides efficiently rather than self peptides.
Firstly, during an inflammatory response, professional antigen presenting cells such as dendritic cells are exposed to factors that enhance the expression of MHC class II on their surface in addition to their ability ...
This solution explains why limiting expression of MHC class II to professional antigen presenting cells is a good strategy for regulating CD4 T cell responses against foreign but not self antigens.