Absent Thymic Shadow and No Humoral Immunity; DGS versus SCID - USMLE Forums
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Old 06-14-2011
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Immunology Absent Thymic Shadow and No Humoral Immunity; DGS versus SCID

From Uworld:

"In DiGeorge syndrome, defective embryonic development of the third and fourth pharyngeal pouches causes thymic aplasia and failure of parathyroid formation. Patients may have recurrent viral and fungal infections due to the T-cell deficiency, but an increased succeptibility to bacterial infections is not characteristic because humoral immunity is normal in these patients"

No thymus = no T-cells = no isotype switching. How can you have intact humoral immunity if you're unable to form anything other than IgM?


Additionally, the vignette described the patient as having an absent thymic shadow on chest X-ray, and the correct answer was SCID. How does that occur? The development of the thymus has nothing to do with the presence of adenosine deaminase or IL-2 receptor, does it?

Thanks as always,
Tig
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Old 06-14-2011
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The description of SCID is correct, there is a B & T cell deficiency either due to (adenosine deaminase / IL 2 Rc ).

What I think it meant by not having recurrent bacterial infections is presence of innate immunity. Neutrophils, Marcophages etc.

They have the ability to phagocytose and kill bacteria through O2/O2 independent mechanisms. Versus virally / tumor infected cells cannot be killed the same way as their DNA has been modified, which required CD4 Help.
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Old 06-14-2011
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Quote:
Originally Posted by patelMD View Post
The description of SCID is corrent, there is a B & T cell deficiency either due to (adenosine deaminase / IL 2 Rc ).
My question is why there is an absent thymic shadow on CXR - adenosine deaminase and IL-2 receptor are not implicated in the development of the thymus from the 3rd pharyngeal pouch

Quote:
Originally Posted by patelMD View Post
What I think it meant by not having recurrent bacterial infections is presence of innate immunity. Neutrophils,Marcophages etc.
I wouldn't be confident to assume that the writers mistook innate immunity for humoral immunity, personally
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Old 06-14-2011
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Quote:
Originally Posted by Tig2575 View Post
No thymus = no T-cells = no isotype switching. How can you have intact humoral immunity if you're unable to form anything other than IgM?
It's always partial T cell deficiency in Digeorge Syndrome. Therefore, you still have class switching and therefore you have normal B cell levels.
Only when T cells are completely gone, you then of course won't have B cells.
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Quote:
Originally Posted by Tig2575 View Post
Additionally, the vignette described the patient as having an absent thymic shadow on chest X-ray, and the correct answer was SCID. How does that occur? The development of the thymus has nothing to do with the presence of adenosine deaminase or IL-2 receptor, does it?
Thymic shadow is not seen after the age of 3 years in most people. So having an absent thymus (on X ray) is not good enough to point you to Digeorge syndrome.
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Old 06-14-2011
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From reading Uptodate, it looks like there is a lot of variability in the phenotype of Digeorge's:

"Clinically, patients with DGS are divided into two subtypes, partial or complete DGS, based upon the level of immunologic function. Partial DGS refers to the majority of patients, who have variable and non life-threatening immunological defects. Complete DGS is a form of SCID, in which there is no apparent thymic tissue and profound immunodeficiency"

So what we think of as DiGeorge's Syndrome is the partial DGS where there is hypoplasia (not absence) of the thymus. So there are some functional T cells but just not enough, so there is a defect in cell mediated immunity.

Complete absence of the thymus would then be considered SCID because like you mentioned, without any T cells, there would be no B cell isotype switching and thus would be fatal.

So for the purposes of USMLE, DiGeorge = small thymus with low T-cell count but humoral immunity is pretty much intact.
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Old 06-14-2011
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Interesting! Thank you all for the responses
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Old 06-15-2011
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thanks really helpful info
hypo= DGS...........aplesia=SCID
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Old 06-15-2011
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when they talk about humoral immunity "may be they" are talking about not t cell dependant immunity such as b cell complete differenciated before ag exposure (moment in which isotype switching start), the abcense of thymus leads to a kind of severe combined immunodeficiency, and IL-2 receptors are in T cells, and there're no T cells because of the abcence of thymus, the question is really confuse, you're right........what does the answer explanation says???.....please let me know....thanks.
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Old 06-15-2011
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thats right thank you so the clue is abcense of thymus or hypoplasia of the thymus, but what if the vignette state NO thymus shadow in xr, as he described it???
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Old 06-15-2011
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Quote:
Originally Posted by yoli View Post
thats right thank you so the clue is abcense of thymus or hypoplasia of the thymus, but what if the vignette state NO thymus shadow in xr, as he described it???
So you either did not read my post above or you don't believe in it.
Here's it again;
Absent Thymic Shadow on X ray does not mean absent thymus.
The thymus normally involutes between ages 2 and 6 years and thence its shadow won't be visible on CXR.

Reference:
The Chest X Ray: A Survival Guide, P. 195
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Old 06-16-2011
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Yes you're right thanks
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