T cells are crucial components of the adaptive immune system, primarily responsible for targeting and eliminating pathogens. Their activation is initiated by antigen presenting cells (APCs), which process pathogens and present their antigens on their surface. A key player in this process is the dendritic cell, a type of APC that interacts with T cells through specific receptors known as T cell receptors (TCRs). These receptors must recognize the presented antigens, which requires a diverse range of TCRs generated through a process called V(D)J recombination, similar to that of antibodies.
The activation of T cells occurs in the thymus, where naive T cells encounter APCs displaying antigens. For a T cell to become fully activated, it must engage with three essential components: the antigen presented by the APC, specific costimulatory molecules, and cell-to-cell adhesion molecules. If any of these interactions fail, the T cell may undergo inactivation or apoptosis, ensuring that T cells are activated only under appropriate conditions.
Once activated, T cells differentiate into various subtypes, each with distinct functions. Cytotoxic T cells (Tc) directly kill infected cells by inducing apoptosis through mechanisms such as perforin, which forms pores in target cells, or through ligands like Fas that trigger cell death. Helper T cells (Th), on the other hand, play a supportive role by activating other immune cells, including B cells and macrophages, thereby amplifying the immune response. Lastly, regulatory T cells (Treg) are essential for maintaining immune homeostasis by suppressing excessive T cell activity and preventing autoimmunity.
In addition to TCRs, T cells express accessory molecules, CD8 and CD4, which are critical for their function. CD8 is found on cytotoxic T cells and interacts with MHC class I molecules, while CD4 is present on helper and regulatory T cells, binding to MHC class II molecules. These accessory molecules stabilize the interaction between T cells and APCs, facilitating effective activation.
In summary, T cells are integral to the adaptive immune response, with their activation and differentiation being tightly regulated to ensure a precise and effective immune reaction against pathogens while preventing autoimmunity.