gpsbrar
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A 4 year old Caucasian boy is brought to his pediatrician with complaints of abnormal bruising and repeated bacterial infections. A blood workup reveals thrombocytopenia and neutropenia and the presence of numerous small, dense lymphoblasts with scant cytoplasm. Immunophenotyping of the abnormal cells determines them to be extremely primitive B cells, which are CD19+, HLA DR+ and Tdt+. Which of the following best describes the status of immunoglobin chain synthesis most likely in these cells?
(a) IgM heavy chains inserted in the membrane
(b) IgM heavy chains present in the cytoplasm
(c) IgM monomers inserted in the membrane
(d) IgM monomers present in the cytoplasm
(e) No Ig chain synthesis present.
Ans is (e). But I could not figure out why? coz it is Tdt+, so it should synthesize heavy chains. Or may be coz it is primitive B cell it is unable to synthesize chains yet.. PLease explain the concept if you can.
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| drk1980
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actually I was going for choice (b)!
Tdt is an enzyme that has a function during heavy chain gene rearrangement. After this step thr is light chain gene rearrangement, and during this time Tdt does not have a function hence will be tdt-ve.
If tdt marker is positive isnt it a sign that the Bcell is stuck in the pre-B cell stage. Heavy chains have been rearranged and formed but not the light chains yet? At this phase doesn't the Pre-B cell have IgM heavy chains in the cytoplasm??
I'm sorry to add to ur confusions! hopefully someone can clarify our doubts!
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| robin082006
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this patient likely has X-linked aammaglubulinemia of Bruton
So no Ig pr0duction
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| drk1980
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my impression was acute lymphoblastic(?) leukemia...the clinical presentation is such. tdt markers are usually positive. also this is the only way i can explain thrombocytopenia.
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| sturge_weber
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well the hint in the q is extremely primitive b cells
and as u must be remembering the b cells go from pro b cells, to pre b cells, to immature b cells,a nd then finally become the mature b cells..
now the tdt is positive in pre b cells , pro b cells, and immature b cells, as these changes re in the marrrow... in pro b cells cytoplasmic mu re present, and in pre b cells surface ig is present and in immature igm is present,,
and as the q says it is very primitive b cells henne no ig will be present
hope u get it
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| msyamp
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may be it is acute lymphoblastic luekemia. so no Ig expression.
not surly brutons hypogammaglobulinemia.
thank you guys. good discussion.
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| sturge_weber
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drk and in the pre b cell stage there re cytoplasmic mu, and surface igm re in the immature b cells, and it says very primitive cells and hence no ig..
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| drk1980
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sturge, i wanted to clarify the 'pre-Bcells have cytoplasmic Mu' and the 'immature cells have surface IgM' right??
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| drk1980
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 thanks!
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| sturge_weber
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yes u re absolutely right drk
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| msyamp
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hey whats up guys? why is it so? why so many smiles?
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| zziggy
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- first a B cell has to make the most complex chain, that is the heavy chain. Once this is done, the light chain is manufactured. (2nd event). The next event is expression of IgM on surface of cell. if it does not over react with self, the B cell will express IgD and will become finally matured - IgM & IgD bothe being expressed and will leave bone marrow.
- cd19, cd 20, cd 21 - all B cell receptors.
- tdt+ means that it's the period when heavy chains are being made (only heavy chains gaind variation with tdt) - so the first DNA recombination event is affected. no Ig chain produced.
E
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