Final answer:
The interaction necessary with the T cell co-stimulatory molecule is the binding between an antigen-displaying MHC molecule and a complementary T-cell receptor.
Step-by-step explanation:
The interaction that is necessary with the T cell co-stimulatory molecule is the binding between an antigen-displaying MHC molecule and a complementary T-cell receptor (TCR). This interaction is important in the activation of T cells to recognize and destroy invading pathogens.
When a CD4+ helper T cell binds the MHC II-antigen complex on an antigen-presenting cell, both the antigen-presenting cell and the T cell release cytokines, which further contribute to the activation of the immune response.