Immunology #8 - T Cell And Cellular Response

T regulatory cells, also known as Tregs, are specialized immune cells that play a crucial role in suppressing autoreactive T cells. Autoreactive T cells are immune cells that mistakenly target and attack the body's own cells and tissues, leading to autoimmune diseases. Tregs help maintain immune tolerance by suppressing the activation and proliferation of autoreactive T cells, thus preventing the development of autoimmune responses. They do this through various mechanisms, including the secretion of immunosuppressive molecules and direct cell-to-cell contact. Overall, T regulatory cells are essential in preventing harmful immune responses and maintaining immune homeostasis.

IL-2 directly drives proliferation of T cells. IL-2 is a cytokine that is produced by activated T cells and plays a crucial role in regulating immune responses. It acts as a growth factor for T cells, promoting their proliferation and survival. IL-2 binds to IL-2 receptors on the surface of T cells, triggering a signaling cascade that leads to cell division and expansion of the T cell population. This cytokine is essential for the immune system's ability to mount an effective immune response against pathogens and tumors.

Positive selection of T cells takes place in the thymus cortex. This is where immature T cells undergo a selection process to ensure that only T cells with functional T cell receptors (TCRs) are allowed to mature and leave the thymus. The thymus cortex provides an environment for T cells to interact with self-antigens presented by specialized cells called thymic epithelial cells. T cells that recognize self-antigens with moderate affinity undergo positive selection, which leads to their maturation and differentiation into functional T cells capable of recognizing foreign antigens.

The "double negative" refers to a stage in T cell development where the cells do not express either CD4 or CD8 co-receptors. During this stage, T cells undergo further maturation and selection processes before they differentiate into CD4 or CD8 single-positive cells. Therefore, the correct answer is "No co-receptors" because double negative T cells do not have either CD4 or CD8 co-receptors.

Dendritic cells are the cells on which B7 is expressed. B7 is a co-stimulatory molecule that plays a crucial role in the activation of T cells. Dendritic cells are antigen-presenting cells that capture, process, and present antigens to T cells, thus initiating an immune response. By expressing B7, dendritic cells provide the necessary co-stimulation to T cells, leading to their activation and subsequent immune response. Therefore, dendritic cells are essential for the initiation and regulation of immune responses.

When the co-stimulatory response is activated without the binding of the MHC:peptide complex, it means that the T cell receptor (CD28) is not being engaged by the antigen-presenting cell. In this case, there will be no effect on the T cell because the co-stimulatory signal alone is not sufficient to activate the T cell. The binding of the MHC:peptide complex is necessary for T cell activation, so without it, the T cell will not be activated.

CD34 is the correct answer because it is a cell-surface marker that is expressed on the stem cell progenitors from which T cells of both lineages originate. CD34 is commonly used as a marker to identify and isolate hematopoietic stem cells, including those that give rise to T cells. It is not expressed on mature T cells, but its expression is critical during early T cell development. CD2, CD28, and CD5 are also cell-surface markers associated with T cells, but they are not specifically expressed on the stem cell progenitors from which T cells originate.