Read the passage. Then answer the questions below. After you have answered the first 10 questions you will answer a 'Summary Question'.
The adaptive immune response, a potent human defense mechanism, is supported by a multitude of cellular actors, the T cell being one of the most important. T cells, also known as T lymphocytes, are essential immune response orchestrators, contributing in a variety of ways, including direct cytotoxic actions, induction of other immune cells, and regulation of immune responses. Due to their immense complexity and diverse responsibilities, a thorough understanding of T cell function provides a lens for comprehending the intricate workings of the immune system.
Beginning in the bone marrow with T cell development, these immune warriors migrate to the thymus for further differentiation, a process significantly influenced by thymic epithelial cells. Their development is strictly regulated, as evidenced by a series of rearrangements in T cell receptor (TCR) gene segments, ensuring a diverse repertoire of T cells capable of combating a vast array of potential pathogens. It is important to note that this diverse T cell pool is self-tolerant, indicating that T cells with an affinity for self-antigens are eliminated through a rigorous process of positive and negative selection in the thymus, thereby preventing autoimmunity.
After leaving the thymus, mature T cells are typically divided into two primary subsets: cytotoxic T cells (CD8+ T cells) and auxiliary T cells (CD4+ T cells). Cytotoxic T cells are adept at identifying and destroying intracellular pathogen-damaged cells, primarily viruses. They achieve this by presenting pathogen-specific peptides within the context of major histocompatibility complex (MHC) class I molecules. Helper T cells, on the other hand, are responsible for assisting B cells in antibody production, facilitating cytotoxic T cell activation, and stimulating innate immune cells. The interaction of their TCRs with pathogen-derived peptides presented by MHC class II molecules mediates their recognition of antigens.
In addition to these two primary subsets, regulatory T cells (Tregs) are an essential component of the T cell milieu. Maintaining immunological homeostasis by dampening excessive immune responses is their primary function. Tregs exert this inhibitory effect by releasing immunosuppressive cytokines, interfering with the metabolic processes of effector T cells, and inducing immune cell apoptosis, among other mechanisms. The equilibrium maintained by Tregs is essential for preventing unwarranted tissue injury and even autoimmunity.
The activation of T cells is a meticulously orchestrated process dependent on a two-signal paradigm. The first signal is the recognition of antigen by TCRs, while the second is the activation of costimulatory molecules. This complex interaction guarantees a high degree of specificity and prevents accidental T cell activation. T cells endure clonal expansion upon effective activation, giving rise to an army of effector cells poised to eradicate the specific pathogen.
The strategic and measured activation and function of T cells are accompanied by a termination strategy. The majority of effector T cells undergo programmed cell death, or apoptosis, following contact with the offending pathogen, thereby preventing an errant immune response. A select few, however, differentiate into memory T cells, which serve as the vanguard against recurring infections. Upon re-exposure to their cognate antigen, these long-lived cells proliferate swiftly and implement effector functions, laying the groundwork for the concept of immunological memory.
We have explained the function of T cells in the human immune response in this lecture. We have emphasized the central role of T cells in orchestrating the body's defense through their complex development, classification into distinct subsets with unique functions, dynamic activation process, and the concept of memory T cells.