Final answer:
Naive T cells, once activated by antigen-presenting cells via MHC molecules, proliferate through clonal expansion and differentiate into various effector cells such as helper T cells and cytotoxic T lymphocytes, which help in directly killing infected cells or in assisting other immune cells.
Step-by-step explanation:
Once naive T cells are activated, they undergo a process known as clonal expansion, where they proliferate and differentiate into various effector cells tailored to address a specific pathogen. For instance, naive CD4+ T cells, after engaging with MHC II molecules on antigen-presenting cells (APCs), become activated helper T cells (TH cells). These TH cells can further differentiate into subtypes such as TH1 cells, which stimulate cell-mediated immunity, and TH2 cells, which aid in humoral immunity by helping B cells. Activated TH cells can also become memory helper T cells, providing a faster response upon subsequent encounters with the same antigen.
On the other hand, naive CD8+ T cells react with MHC I molecules on APCs, leading them to become cytotoxic T lymphocytes (CTLs). These CTLs have the primary function of directly killing infected cells through induced apoptosis. Both CD4+ and CD8+ cells use their T cell receptors (TCRs) in conjunction with their respective coreceptors, CD4 and CD8, to interact effectively with MHC complexes and eliminate threats.