Final answer:
Helper T cells are activated by two simultaneous events: the TCR's recognition of a specific foreign epitope and the interaction of CD4 with MHC II, followed by cytokine release leading to activation, proliferation, and differentiation into subtypes.
Step-by-step explanation:
The activation of helper T cells involves two simultaneous events: TCR recognition of a specific foreign epitope presented within the MHC II antigen-binding cleft and the interaction of CD4 with MHC II. Following recognition and anchoring of the TCR and CD4 to MHC II, cytokines released by both the antigen-presenting cell (APC) and the helper T cell lead to T cell activation.
This activated helper T cell then proliferates and can differentiate into various subtypes, such as TH1, TH2, or memory helper T cells, each with different roles in the immune response.