What are the selection processes in thymus?

There are two main selection processes in the thymus – positive thymic selection and negative thymic selection, both of which are necessary for successful T cell maturation. The selection process is important in shaping the thymocyte population into T cells that can appropriately respond to foreign pathogens but remain tolerant towards the body’s own antigens.

During positive selection, immature T-cells, or thymocytes, that are capable of recognizing MHC molecules are favored since MHC recognition is essential for T-cell function. If T-cell clones fail to achieve this basic ability, then they will undergo apoptosis.

After passing positive thymic selection, the T-cells that remain are capable of recognizing MHC molecules. The aim of negative thymic selection is to eliminate those T-cell clones that are capable of recognizing self-antigens presented by MHC. MHC itself does not distinguish between self and foreign antigens. This differentiation is made by the T cells and their receptors as a result of negative thymic selection. 

Only about 2% of the original thymocytes survive the dual selection process. Resulting from the positive and negative selection processes are populations of T-cell clones, each with the potential to recognize various exogenous or foreign antigens as complexed with MHC, but not self antigens.