Final answer:
The second stimulatory signal needed for T cell activation consists of the interaction of CD4 with the MHC II molecule and the secretion of cytokines by both the APC and the T cell, further activating the T cell.
Step-by-step explanation:
The second stimulatory signal required for T cell activation in the immune response to a T cell-dependent antigen involves not only the presentation of the foreign antigen in association with an MHC II molecule, but also the interaction of CD4 on the helper T cell with a part of the MHC II molecule. This interaction solidifies the recognition of both the foreign antigen and the "self" antigen of the Antigen-Presenting Cell (APC). In addition to this, the APC and the helper T cell both secrete cytokines as part of this second signal, which further activate the helper T cell. After activation, the helper T cell can participate in the immune response by proliferating and differentiating into subtypes that can either assist other immune cells, like B cells, or work directly in combating pathogens.