A 19-year-old man comes to the physician due to diarrhea for the last 4 months. He visited a student health clinic at his university a month ago and was prescribed a course of antibiotics, but his diarrhea failed to improve. He also has persistent nausea, a bloating sensation, and a 6.8-kg (15-lb) weight loss. Microscopic examination of his stool for ova and parasites following trichrome staining is shown in the exhibit.
Impairment of which of the following immune functions would most likely predispose to this patient's infection?

Question

A 19-year-old man comes to the physician due to diarrhea for the last 4 months.  He visited a student health clinic at his university a month ago and was prescribed a course of antibiotics, but his diarrhea failed to improve.  He also has persistent nausea, a bloating sensation, and a 6.8-kg (15-lb) weight loss.  Microscopic examination of his stool for ova and parasites following trichrome staining is shown in the exhibit. 
 Impairment of which of the following immune functions would most likely predispose to this patient's infection?

Quizplus · Accepted Answer

11ec160b_386e_f624_9249_091ab7e8b084_MD0001_00 Giardia lamblia is an intestinal flagellate that exists in 2 forms: a trophozoite (pathogenic stage) or a cyst (infective stage). The trophozoite is a bilaterally symmetric, pear-shaped organism with multiple flagella and 2 nuclei (owl's eyes appearance); the cysts are oval and contain up to 4 nuclei. The disease is transmitted by drinking contaminated water (eg, campers, travelers to endemic areas) or via fecal-oral transmission (eg, children in day care centers). Giardiasis can be asymptomatic or present with a variable combination of diarrhea, nausea, abdominal cramps, and fat malabsorption (eg, foul-smelling stools). The diagnosis is confirmed by stool microscopy for ova and parasites or fecal immunoassays for Giardia antigens. Small-bowel biopsy is sometimes performed when stool studies are nondiagnostic and can show varying degrees of villous atrophy and crypt hyperplasia depending on disease severity. Immune defence against Giardia involves CD4+ T helper cell induction of secretory IgA production. Secretory IgA helps prevent and clear infection by binding to trophozoites and impairing their adherence to the upper small-bowel mucosa. Children with IgA deficiency, X-linked agammaglobulinemia, and common variable immune deficiency have a predisposition to developing chronic giardiasis. (Choices A and D) Neutrophils and the complement system are important defense mechanisms against bacterial infections. They have no significant role in controlling parasitic infections. (Choice B) CD8+ T cells are not important in clearing giardiasis; they are more important for combating intracellular intestinal pathogens such as Cryptosporidium parvum and Toxoplasma gondii. (Choice C) Eosinophils do not play an important role in immune defense against Giardia; eosinophilia is not seen in giardiasis and white cells are usually absent from the stool. Eosinophils are more important for combating intestinal helminthic infections. Educational objective: Giardia lamblia causes injury to the duodenal and jejunal mucosa by adhering to the intestinal brush border and releasing molecules that induce a mucosal inflammatory response. Secretory IgA, which impairs adherence, is the major component of adaptive immunity against G lamblia infection. Conditions causing IgA deficiency predispose patients to chronic giardiasis.

A 19-Year-Old Man Comes to the Physician Due to Diarrhea