> See also: > - Reference # The Thymus For more information about the thymus see [[T-Cell Development]] 1. Subcapsular Region 2. Cortex 3. Cortico-Medullary Junction 4. Medulla ## T-Cell Development While(and all blood cells) are created within the bone marrow Keep in mind that [[T-Cells]] are: - Developed in the bone marrow - Activated The development of T-Cells occurs in [[the thymus]]. While these cells are maturing they are referred to as **thymocytes**. There are two stages to their development: - **Positive Selection:** Ensures the new TCRs can recognize one of your [[Major Histocompatibility Complex (MHC)|MHCs]] - **Negative Selection:** Ensures the TCRs will not recognize your "self" [[Major Histocompatibility Complex (MHC)|MHCs]] and peptides too strongly and become activated > Roughly 70% of your thymocytes will If a T-Cell fails the maturation process, it will kill itself via [[Apoptosis]] ![[T-Cell Development.png|300]] At the beginning of the maturation process, T-Cells contain both CD4 and CD8 receptors on their membrane ## Positive Selection > - [[Major Histocompatibility Complex (MHC)]] At the beginning of the maturation process, T-Cells contain both CD4 and CD8 receptors on their membrane If a thymocyte successfully recognizes either an MHC Class I or II molecule, it will receive survival and maturation signals If an *MHC Class I* molecule is recognized: 1. Receive survival and maturation signals 2. Stop expressing **CD4 receptors** 3. Maintains **CD8 receptor** expression If an *MHC Class II* molecule is recognized: 1. Receive survival and maturation signals 2. Stop expressing **CD8 receptors** 3. Maintain **CD4 receptor** expression Any thymocyte that receives a survival signal during the training process is said to undergo **positive selection** This process occurs in the **thymic cortex** region of [[The Thymus]] where **cortical epithelial cells** reside. ### Double Positive At the double-positive stage, thymocytes rearrange α-, γ-, and δ-chain genes. Only one productive α-chain rearrangement is needed for commitment to the α:β T-cell lineage. ## Negative Selection ### Double Negative At the double-negative stage of T-cell development, thymocytes rearrange β-, γ-, and δ-chain genes. Only one productive β-chain rearrangement is needed to progress to the double-positive stage.