WBR0494

Author	[[PageAuthor::Rim Halaby, M.D. [1] (Reviewed by Serge Korjian and Alison Leibowitz [2])]]
Exam Type	ExamType::USMLE Step 1
Main Category	MainCategory::Immunology
Sub Category	SubCategory::Endocrine
Prompt	[[Prompt::A 27-year-old newly graduated nurse is applying for a job at a hospital. As part of her pre-employment screening tests, she has to submit a copy of her vaccination history and undergo a PPD test to screen for tuberculosis. She is administered 0.1 mL of 5 tuberculin units intradermal injection and is asked to return 48 hours later. Upon re-evaluation, the physician notices a 12 mm induration at the site of injection. Which of the following mechanisms is most likely responsible for this finding?]]
Answer A	AnswerA::Hypersensitivity type I
Answer A Explanation	[[AnswerAExp::Type I hypersensitivity disorders include anaphylaxis, atopy, and allergy. A PPD skin test is not a type I hypersensitivity immune reaction.]]
Answer B	AnswerB::Hypersensitivity type II
Answer B Explanation	AnswerBExp::Type II hypersensitivity disorders include conditions where an antibody is directed against a specific antigen. A PPD skin test involves a delayed T cell mediated response that is consistent with type IV hypersensitivity.
Answer C	AnswerC::Hypersensitivity type III
Answer C Explanation	AnswerCExp::Hypersensitivity type III disorders include conditions where an antigen-antibody complex forms and activates the complement system. A PPD skin test is not a type III hypersensitivity immune reaction.
Answer D	AnswerD::Hypersensitivity type IV
Answer D Explanation	[[AnswerDExp::Hypersensitivity type IV disorders include conditions characterized by delayed T-cell mediated immune reactions, such as contact dermatitis and tuberculosis testing.]]
Answer E	AnswerE::Not a hypersensitivity disorder
Answer E Explanation	AnswerEExp::PPD skin test is a type IV hypersensitivity disorder.
Right Answer	RightAnswer::D
Explanation	[[Explanation::PPD test, also known as the Mantoux test, tuberculin sensitivity test, and Pirquet test, is a screening tool for tuberculosis. The test is carried out by administering 5 tuberculin units as an intradermal injection on the volar surface of the forearm and monitoring the injection site for the development of induration within 48 to 72 hours. The individual's medical risk factors determine the increment (5 mm, 10 mm, or 15 mm) of induration at which the result is considered positive. A PPD test, a type IV hypersensitivity reaction, is characterized by delayed T-cell mediated immune response. The tuberculin antigen is phagocytosed and presented to CD4+ lymphocytes via MHC II. CD4+ Th1 cells recognize the tuberculin antigen and release IL-12, IFN-gamma that activate CD8+ cytotoxic T-cells. This reaction leads to the induration observed. Educational Objective: PPD testing is a classic example of a type IV hypersensitivity. References: Beck, JS. Skin changes in the tuberculin test. Tubercle. 1991;72(2):81-87. First Aid 2014 page 208]]
Approved	Approved::Yes
Keyword	WBRKeyword::Hypersensitivity, WBRKeyword::Type IV hypersensitivity, WBRKeyword::PPD test, WBRKeyword::Immune system, WBRKeyword::Testing, WBRKeyword::TB, WBRKeyword::Tuberculosis, WBRKeyword::Screening
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