Part 3/8:
Once mature, naive T cells exit the thymus and enter peripheral lymphoid tissues such as the spleen and lymph nodes. Here, they encounter foreign peptides presented by antigen-presenting cells (APCs)—including macrophages, dendritic cells, and B cells—via major histocompatibility complex (MHC) molecules.
The engagement of the TCR with peptide-MHC complexes triggers T cell activation. This process involves clonal expansion and differentiation into effector cells capable of orchestrating targeted immune responses. Effective TCR signaling, therefore, is foundational not only to T cell development but also to their activation during immune challenges.