Review Question - QID 109803

A 21-year-old female presents to the emergency room complaining of two weeks of diarrhea and abdominal pain. She also experienced multiple episodes of hematuria the day prior to presentation. She is a refugee from South Sudan who arrived in the United States one month ago. She has no known past medical history and takes no medications. She is sexually active and does not use contraception. Her temperature is 100.1°F (37.8°C), blood pressure is 135/75 mmHg, pulse is 110/min, and respirations are 20/min. On exam, mild hepatosplenomegaly is noted. She has no costovertebral angle tenderness. A pregnancy test and urinalysis are negative. Results of a complete blood count are shown below:

Leukocyte count and differential:
Leukocyte count: 13,000/mm^3
Segmented neutrophils: 54%
Bands: 5%
Eosinophils: 8%
Basophils: 0.5%
Lymphocytes: 29%
Monocytes: 4%

Results of a stool analysis are shown in Figure A. Which of the following is most responsible for mediating the immune response in this patient?