The first step of thymic selection occurs in the cortex of the thymus and involves the development of a functional T-cell receptor that is required for activation by APCs. Thymocytes with defective TCRs are removed by negative selection through the induction of apoptosis.

The First Step of Thymic Selection

The second step of thymic selection also occurs in the cortex and involves the positive selection of thymocytes that will interact appropriately with MHC molecules. Thymocytes that interact appropriately with MHC molecules receive a positive stimulation that moves them through the process of maturation, and thymocytes that do not interact appropriately  are eliminated by apoptosis

The Second Step of Thymic Selection 

The third step of thymic selection occurs in the cortex and medulla. It involves negative selection to remove self-reacting thymocytes by apoptosis. This step is also referred to as central tolerance because it prevents self-reacting T cells from reaching the bloodstream and potentially causing autoimmune disease.

The Third Step of Thymic Selection 

Thymic selection is the process of thymocytes maturing within the thymus. It can be divided into three critical steps of positive and negative selection:
- The First Step of Thymic Selection
- The Second Step of Thymic Selection 
- The Third Step of Thymic Selection

Austin Community College

Thymocytes are immature T lymphocytes in the thymus. While they are in the thymus they undergo thymic selection. 

Thymocytes

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