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A 7-year-old girl is suspected of being infected with Mycobacterium tuberculosis. She develops a positive skin test reaction to intradermal injection with purified protein derivative (PPD). Which of the following best describes the mechanism underlying the reaction to the PPD injection?

A. Immunoglobulin E (IgE) bound to mast cells triggers mast cell activation after the IgE binds PPD.

B. Immunoglobulin G (IgG) binds to PPD, initiating an influx of neutrophils and macrophages.

C. Immune complex formation in the walls of blood vessels triggers complement activation and blood vessel damage.

D. Antigen-presenting cells ingest PPD and present antigen to T cells, thereby triggering T-cell activation to induce an influx of macrophages and other inflammatory cells.

E. Direct activation of complement by PPD results in C5a production and influx of neutrophils.

Answer: D

The PPD test is based on type IV hypersensitivity in which resident antigen-presenting cells (eg, macrophages and dendritic cells) internalize, process, and present the PPD antigenic epitopes in the surface class II major histocompatibility complex (MHC) molecule to T cells. T cells that have been previously sensitized by exposure to M tuberculosis during infection recognize the antigen in the MHC molecule and are activated, releasing cytokines that further activate macrophages. The activated macrophages release cytokines, such as tumor necrosis factor (TNF) and interleukin (IL)-1, that upregulate molecules on the endothelium, such as the selectins, and induce increased blood flow and vascular leak in the area. The selectins traffic inflammatory cells into the site of the injection. These inflammatory cells and the edema resulting from vascular leak cause the characteristic induration and erythema at 24 to 72 hours after the injection.

Answer choices A, B, and C are descriptions of type I, II, and III hypersensitivity, respectively, whereas answer choice D is a description of type IV hypersensitivity. Some substances and bacteria activate complement directly, but PPD does not.

A 10-year-old boy is being treated for a hypocalcemic seizure and is found to have hypoparathyroidism. He has suffered from chronic thrush and onychomycosis due to Candida albicans since he was 1 year of age. In addition, he has had episodes of a recurrent maculopapular rash described as nonpruritic urticaria since 2 years of age. His younger sister also has chronic mucocutaneous candidiasis. His parents are not affected by any of these disorders. Genetic testing of the siblings reveals that they both have biallelic mutations in the AIRE gene, which are known to cause loss of function in the protein product of this gene. A diagnosis of autoimmune regulator and autoimmune ...

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