Final answer:
The second step of T cell activation is co-stimulation, which is a vital interaction between CD4/CD8 on the T cell and MHC proteins on the APC.
Step-by-step explanation:
The second step of T cell activation involves the interaction of CD4 or CD8 on the T cell with MHC proteins on the antigen-presenting cell (APC) - specifically, co-stimulation. For helper T cells, the interaction between CD4 and MHC II is essential, whereas for cytotoxic T cells, the interaction is between CD8 and MHC I. This process is critical for ensuring that the T cell is recognizing both the foreign ("nonself") epitope and the "self" antigen presented by the APC, which are both required for proper activation of the T cell.