- T cells, or T lymphocytes, are a critical component of the adaptive immune system, playing a central role in cell-mediated immunity.
- They originate in the bone marrow as hematopoietic stem cells but, unlike B cells, migrate to the thymus gland to mature, which is the reason for the “T” designation.
- During their time in the thymus, T cells undergo a rigorous selection process to ensure they can recognize self from non-self and are capable of mounting an effective immune response without attacking the body’s own tissues. This selection process is crucial in preventing autoimmune diseases.
- Once mature, T cells circulate throughout the bloodstream and lymphatic system, patrolling for threats. They are essential for recognizing and responding to specific antigens by using T cell receptors (TCRs) on their surface to identify and bind to antigens presented by other cells.
- When a T cell encounters an antigen presented by an antigen-presenting cell (APC), such as a dendritic cell or macrophage, it becomes activated. This activation triggers a cascade of events, leading to the differentiation of T cells into various subtypes with distinct functions.
- Among the most important T cell subtypes are helper T cells (Th cells) and cytotoxic T cells (Tc cells).
- Helper T cells play a crucial role in orchestrating the immune response. When they encounter an antigen, they release signaling molecules called cytokines that help activate and direct other immune cells, such as B cells, cytotoxic T cells, and macrophages. This coordination ensures a robust and effective immune response to pathogens.
- Cytotoxic T cells, also known as killer T cells, are responsible for directly attacking and destroying infected or cancerous cells. Upon recognizing an antigen on the surface of a target cell, they release cytotoxic granules containing perforin and granzymes. Perforin creates pores in the target cell membrane, allowing granzymes to enter and induce apoptosis, or programmed cell death. This precise mechanism helps eliminate infected cells while minimizing damage to surrounding healthy tissue.
- Another important group of T cells includes regulatory T cells (Tregs), which help prevent the immune system from attacking the body’s own tissues. Tregs maintain immune tolerance and prevent autoimmune diseases by secreting inhibitory cytokines, such as interleukin-10 (IL-10) and transforming growth factor-beta (TGF-β), and by directly interacting with other immune cells to modulate their activity.
- Memory T cells, both CD4+ and CD8+, provide long-lasting immunity by “remembering” past encounters with specific antigens, allowing for a rapid and robust response upon re-exposure. After an infection has been cleared, some T cells remain as memory T cells, ready to mount a faster and stronger response if the same pathogen is encountered again. This ability to “remember” previous invaders is fundamental to how vaccines work, providing protection against future infections.
- The role of T cells is essential for defending the body against a wide variety of diseases, including infections and cancers, making them indispensable for maintaining immune homeostasis.
