The patient has common variable immunodeficiency disorder (CVID), which leads hypoagammaglobulinemia. Specifically, the patient has a deficiency in IgA, therefore he has an increased susceptibility to Giardia infection.
IgA plays an important role in mucosal immunity. By its presence on the surface of intestinal mucosal cells, IgA is able to prevent the binding of pathogens to the epithelial cells; thus, preventing protozoa like Giardia lamblia from causing inflammation. Its absence, therefore, leads to the increased likelihood of repeat infection of the GI mucosa and repeat episodes of non-bloody diarrhea.
Kucik et al. review common intestinal parasites: Giardia often presents with nausea, vomiting, diarrhea and potentially weight loss and is diagnosed with stool studies for ova and parasites. Metronidazole (Flagyl) is the treatment of choice. Infection can be prevented by proper hand sanitation, upkeep of sewage, and bottled water in place of contaminated sources.
Cooper et al. examine primary immunodeficiences and note how CVID is the most common primary immunodeficiency found in the US. The authors describe the major manifestation of CVID as the presence of hypogammaglobulinemia, often with low serum IgA levels.
Illustration A shows a light microscopic view of Giardia lamblia.
Answer 1: CD8+ T cells play a prominent role in the killing of infected cells.
Answer 2: Neutropenia is most commonly a result of drug toxicity, but the patient is not on any medications, therefore, low neutrophil count is unlikely.
Answer 4: NK cells play a prominent role in viral immunity.
Answer 5: Though the patient is likely deficient in IgG, the recurrent episodes of diarrhea (from G. lamblia) indicate it is not the only immunoglobulin absent.
Kucik CJ, Martin GL, Sortor BV. Common intestinal parasites. Am Fam Physician. 2004 Mar 1;69(5)
PMID:15023017 (Link to Abstract)
Cooper MA, Pommering TL, Korányi K. Primary immunodeficiencies. Am Fam Physician. 2003 Nov 15;68(10):2001-8
PMID:14655810 (Link to Abstract)