The Question is from neet-zoology chemical-coordincation-and-integration thymus-gland, and it was created by the author T HARITHA

Question

A patient presents with recurrent infections, severely depleted T-cell counts, and underdeveloped secondary lymphoid organs. Genetic analysis reveals a mutation affecting a specific receptor crucial for thymocyte development. Which receptor is MOST likely mutated, leading to this severe immunodeficiency?

T HARITHA · Accepted Answer

## CORE INFORMATION
✓ **Answer**: c
## CONCEPT FRAMEWORK
 **Primary Concept**: T-cell development and the role of Notch1 signaling in the thymus.
 **Related Topics**: Immunodeficiency disorders, thymocyte maturation, receptor signaling pathways, lymphoid organogenesis.
 **Prerequisites**: Basic understanding of the immune system, T-cell development stages (double negative, double positive, single positive), and the role of key receptors in these processes.
## SOLUTION PATHWAY
**Given Information**:
  * Recurrent infections indicating compromised immunity.
  * Severely depleted T-cell counts (T-lymphocytopenia).
  * Underdeveloped secondary lymphoid organs (e.g., lymph nodes, spleen) where T-cell maturation and interactions occur.
  * Mutation in a receptor crucial for thymocyte development.
**Method & Steps**:
 1. Identify the key process affected: The symptoms point towards a defect in T-cell development within the thymus.
  Reasoning:  Recurrent infections and low T-cell counts directly indicate an impaired immune response, specifically the cellular arm mediated by T-cells. Underdeveloped secondary lymphoid organs suggest a problem originating early in T-cell development, likely in the thymus where T cells mature.
 2. Consider the receptors involved in thymocyte development: Several receptors play critical roles in thymocyte development, including IL-7R, Notch1, CXCR4, and CCR7.  Each has specific functions at different stages.
  Working:  The Notch1 receptor is essential for the transition from the double-negative to the double-positive stage of T-cell development in the thymus.  Disruptions in Notch1 signaling lead to a significant block in T-cell development.  IL-7R is involved in early thymocyte survival and proliferation but its mutation does not typically lead to such severe immunodeficiency. CXCR4 and CCR7 are involved in thymocyte migration but their mutations are less likely to cause such a profound developmental defect and low T cell counts.
 3. Determine the most likely mutated receptor: Given the severity of the immunodeficiency (severe T-cell depletion and underdeveloped lymphoid organs), the most likely culprit is a mutation affecting Notch1, which is critical for early thymocyte development. This would result in a failure of T-cell differentiation and maturation, leading to the observed clinical picture.
  Result: Notch1 receptor
  Verification: The clinical presentation strongly suggests a defect in early T-cell development, and Notch1 signaling is central to this process.  The other options play less crucial roles in the overall number and maturation of T-cells.
 ## OPTION ANALYSIS
A :x: IL-7 receptor is important for thymocyte survival and proliferation, but its deficiency alone wouldn't cause such profoundly underdeveloped secondary lymphoid organs.
B :x: CXCR4 receptor is involved in the migration of thymocytes, but its deficiency would not lead to such a severe block in T cell development.
C ✓: Notch1 receptor is essential for the transition from DN to DP thymocytes.  Its mutation severely impairs T-cell development, leading to the observed immunodeficiency.
D :x: CCR7 receptor is important for the migration of mature T cells to secondary lymphoid organs, but its deficiency would not lead to severely underdeveloped secondary lymphoid organs.
 ## LEARNING AIDS
 **Common Mistakes**:
  1.Confusing the roles of different receptors involved in T-cell development: Each receptor has specific functions at different stages. Understanding these distinctions is crucial.
  2.Overlooking the severity of the clinical picture: The severe immunodeficiency points to a fundamental defect in early thymocyte development, not just a minor disruption in migration or survival.
 **Quick Tips**:
  1.Remember the key role of Notch1 signaling in the DN to DP transition.
  2.Associate severe combined immunodeficiency (SCID) with defects in early lymphocyte development, often involving crucial signaling pathways like Notch1.

T HARITHA · Answer

a) IL-7 receptor

T HARITHA · Answer

b) CXCR4 receptor

T HARITHA · Answer

The correct Option is "c) Notch1 receptor"

T HARITHA · Answer

d) CCR7 receptor

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