We have the two classic choices.
P acts as a receptor.
Of course it is also possible to imagine the TCR undergoing a weak conformational change upon engaging R, and a strong aggregation reaction if the TCR also engages the peptide. What is important is that these are about the only two choices available, and experiments will eventually sort them out. This is the story of the APC or antigen presenting cell. Recall that RII is uniquely expressed on lymphoid cells whereas RI is expressed on all cells, including lymphocytes. This is now obvious on the grounds that there is no point to having T cells regulate all the cells of the body when only the lymphoid cells are appropriate targets.