Final answer:
During T cell development, there is a stage called double negatives where thymocytes do not express CD4 or CD8 molecules. These cells are exposed to cortical epithelial cells and undergo positive selection. The remaining T cells will become either CD4+ helper T cells or CD8+ cytotoxic T cells.
Step-by-step explanation:
In the process of T cell development, there is a stage called double negatives where the thymocytes do not express CD4 or CD8 molecules (also known as double negatives). These cells are found in the cortex of the thymus and are exposed to cortical epithelial cells.
During positive selection, the double-negative thymocytes bind to MHC molecules they observe on the thymic epithelia, and the MHC molecules of 'self' are selected. This leads to the death of many thymocytes. The remaining T cells, those that can bind to MHC molecules of the body with foreign antigens, will become either CD4+ helper T cells or CD8+ cytotoxic T cells.