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NBME 20 Answers

nbme20/Block 3/Question#4

An 18-year-old woman comes to the physician because ...

Reactive T lymphocytes

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FA 2017: Infects B cells through CD21. Atypical lymphocytes on peripheral blood smear G —not infected B cells but reactive cytotoxic T cells. ⊕ Monospot test—heterophile antibodies detected by agglutination of sheep or horse RBCs. Use of amoxicillin in mononucleosis can cause characteristic maculopapular rash.

zup  misread the "accounts for" question as what's the reason for the atypical lymphocytes. So I answered "virus infected B lymphocytes." Anyone else misread it like that? +4  
nala_ula  Shit, I misread that too and I noticed it now. Nerves get the best of us! +  
stevenorange  If the question is ask what is the atypical lymphocyte in the brain , than it should be the infected B cell, RIGHT? +  

These atypical lymphocytes, aka Downey cells (Downey 1923), are actually activated CD8 T lymphocytes, most of which are responding to EBV-infected cells.

CD8+ T- cells increase in numbers in the blood stream and are activated (aka Downey cells, reactive lymphocytes or atypical lymphocytes because of their atypical presence in peripheral blood) to eliminate EBV infected B- lymphocytes.

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submitted by katsu(-1),

Patient has infectious mono. Atypical lymphocytes (reactive CD8+ T-cells) are seen on blood smear for infectious mono.