Here's what I learned about how regulatory T cells work:
There is a self-recognising naive T cell around.
A regulatory T cell that recognises the same self-antigen binds the same antigen-presenting cell as one of these T cells.
It suppresses activation of the naive T cell.
However, let's say the antigen-presenting cell is presenting foreign antigen on some of its MHC molecules. The naive T cell binds the antigen and there happens to be a regulatory T cell nearby binding a self-antigen on the same antigen-presenting cell.
Does the regulatory T cell suppress the naive T cell? Is it just do with proximity, or do both cells have to bind the exact same antigen?