What happens in the cortex and medulla of the thymus to produce naïve T cells
The correct answer and explanation is:
The thymus is essential for the development of naïve T cells, which are critical for the adaptive immune system. It consists of two main regions: the cortex and the medulla. Both regions play distinct roles in T cell development.
In the cortex of the thymus, immature T cells, or thymocytes, originate from hematopoietic stem cells in the bone marrow. These cells enter the thymus and undergo a process of positive selection. The thymocytes express T cell receptors (TCRs) and interact with thymic epithelial cells that present self-antigens. During positive selection, the T cells that can recognize and bind to the self-antigens presented by these epithelial cells are selected to continue developing. However, those T cells that cannot bind to self-antigens undergo apoptosis (programmed cell death). The ones that bind with an appropriate affinity to the self-antigens are allowed to mature and proceed to the next stage.
In the medulla of the thymus, the developing T cells undergo negative selection. In this step, the thymocytes that react too strongly to self-antigens presented by medullary thymic epithelial cells or dendritic cells are eliminated. This process is crucial because it prevents the development of autoimmunity, ensuring that T cells that could potentially attack the body’s own tissues are removed.
After both positive and negative selections, the surviving T cells exit the thymus as naïve T cells. These naïve T cells have undergone a rigorous maturation process to ensure that they can recognize foreign pathogens while avoiding attacks on the body’s own cells. Once matured, they migrate to secondary lymphoid organs like lymph nodes and spleen, where they are ready to be activated by specific pathogens. These naïve T cells circulate in the bloodstream, awaiting activation by an antigen-presenting cell that triggers their response.