Cytotoxic T- lymphocyte Negative selection Thymus Cortex of thymus Helper T- lymphocyte Recognizes self- antigen Sel antigen TCR MHC class Positive selection -CDs CD4 Thymic epithelial cell presents MHC Dendritic cell presents self antigen Thymocyte Binds to MHC molecule

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Cytotoxic T- lymphocyte Negative selection Thymus Cortex of thymus Helper T- lymphocyte Recognizes self- antigen Sel antigen TCR MHC class Positive selection -CDs CD4 Thymic epithelial cell presents MHC Dendritic cell presents self antigen Thymocyte Binds to MHC molecule? No Destroyed by apoptosis No Survives 2% of cells survive selection process in the thymus Selective loss of either CD4 or CD8 proteins T lymphocytes retain only CD4 protein Retains only CD8 protein

The Correct Answer And Explanation is:

To properly label the figure described in your request, we need to understand the T-cell maturation and selection process in the thymus, where immature thymocytes develop into functional T lymphocytes. Here’s how the elements you listed fit into the flow and labeling:

Correct Labeling (based on the process):

  1. Thymus – The organ where T-cell maturation occurs.
  2. Cortex of thymus – Site of positive selection.
  3. Thymocyte – Immature T cell.
  4. TCR (T-cell receptor) – Present on thymocytes to recognize antigens.
  5. MHC class I and II molecules – Presented by thymic epithelial cells (for positive selection).
  6. Thymic epithelial cell presents MHC – In cortex, presenting self-MHC to thymocytes.
  7. Binds to MHC molecule?
  • Yes → Survives
  • No → Destroyed by apoptosis
  1. Positive selection – Occurs in the cortex, ensures thymocytes can recognize self-MHC.
  2. Selective loss of either CD4 or CD8 proteins – Based on MHC recognition.
  • MHC Class I → Retains CD8 → Cytotoxic T-lymphocyte
  • MHC Class II → Retains CD4 → Helper T-lymphocyte
  1. Medulla of thymus – Site of negative selection.
  2. Dendritic cell presents self-antigen – In medulla.
  3. Recognizes self-antigen?
    • Yes → Destroyed by apoptosis
    • No → Survives
  4. Negative selection – Removes self-reactive T cells.
  5. 2% of cells survive selection process in the thymus – Final surviving mature T cells.

300-Word Explanation:

T-cell maturation is a critical immunological process occurring in the thymus, ensuring the development of functional but non-self-reactive T lymphocytes. Immature T cells, or thymocytes, enter the cortex of the thymus where they undergo positive selection. This stage tests whether thymocytes can recognize self-MHC molecules using their T-cell receptor (TCR). If the thymocyte fails to bind MHC, it undergoes apoptosis. If it binds appropriately, it survives and progresses.

After passing positive selection, thymocytes undergo lineage commitment:

  • If a thymocyte binds to MHC class I, it downregulates CD4 and retains CD8, becoming a cytotoxic T lymphocyte.
  • If it binds MHC class II, it retains CD4, becoming a helper T lymphocyte.

Next, in the medulla, negative selection occurs. Here, dendritic cells and medullary thymic epithelial cells present various self-antigens. Thymocytes that strongly recognize self-antigens are eliminated via apoptosis, preventing autoimmunity.

This two-step selection process is highly stringent; only about 2% of all thymocytes survive both tests. These surviving T cells exit the thymus as mature, self-MHC-restricted and self-tolerant lymphocytes. This mechanism ensures a competent and safe adaptive immune response, crucial for defending the body against pathogens while preventing attacks on self-tissues.

In summary, labeling the figure with the correct terms reflects the tightly regulated development of T cells through positive and negative selection in the thymus, forming the foundation of adaptive immunity.

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