Final answer:
APCs activate T lymphocytes by presenting antigens with MHC molecules. Cytotoxic T cells are activated by recognition of antigens presented with MHC I and interaction with CD8. Helper T cells are activated by recognition of antigens presented with MHC II and interaction with CD4.
Step-by-step explanation:
Cytotoxic T cells (CTLS) are activated by antigen-presenting cells (APCs) in a three-step process. The key difference is that the activation of cytotoxic T cells involves recognition of an antigen presented with MHC I and interaction of CD8 with the receptor complex. After co-recognition of foreign epitope and self-antigen, the production of cytokines by the APC and the cytotoxic T cell activate proliferation and differentiation. Activated cytotoxic T cells can differentiate into effector cytotoxic T cells or memory cells.
Helper T cells, on the other hand, can only be activated by APCs presenting processed foreign epitopes in association with MHC II. The activation process involves TCR recognition of the foreign epitope presented within the MHC II antigen-binding cleft and the interaction of CD4 on the helper T cell with a region of the MHC II molecule. The second interaction anchors the MHC II-TCR complex and activates the helper T cell. The activated helper T cell then proliferates and differentiates into subtypes with different functions.
The activation of T cells is essential for the acquired immune response. T and B cells are activated when they recognize specific epitopes presented by APCs. Without information from APCs, T and B cells remain inactive and cannot prepare an immune response.