NBME 15 T-lymphocyte thymocytes localized to the thymic cortex (NBME Answers)

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submitted by ∗andro(269)

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First question : They tell us its a lymphocytic leukemia

Second question : Deciphering whether it is a B cell or T cell.

The cells have no surface bound Ig M ,Ig G or cytoplasmic chains for these antibodies , but have a rearranged gene segment( meaning active ) for the T cell Receptor . So the Cells are T Cells

Third question : Now that we know that they are T Cells , we want to know whether they are CD4 or CD 8 positive

The cells in the clinical vignette lack both CD4 and CD 8 ( double negative ) According to how T cells normally develop - They start of as being double negative ( i.e not expressing CD4 or CD 8) in the subcapsular region and progressively become
- positive for both CD 4 and CD 8 in the cortex
- and eventually acquire the TCR and CD3 by the time they get to the medulla

These cells therefore must T lymphocyte thymocytes localised to the cortex

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i_hate_it_here FA2020 pg: 102 +1

dawgtor Don't cells in the thymic cortex express both CD4 and CD8? That threw me off a bit. I went with activated cytolytic effector T cells because i thought they would bind MHC I. +

makingstrides Yeah, that's what I chose too @dawgtor. But I think when the cells are made in the bone marrow, they don't have the MHC classes/ TCR either. When they get into the thymic cortex, they gain those and undergo positive selection. This is from FA2020 page 102. Correct me if I am wrong. +1

victorlt14 The thymocyte (T cell precursor, not T cell quite yet) gets in the subcapsular zone of the cortex with no CD4/CD8. It took me longer to be sure it was a cell of T lineage though. Looking back I think it should've been rather easy to recognise that since the cell line has a T cell receptor it is of T cell lineage. +