In individuals with IgA deficiency, which infection is most commonly seen?

• A. Streptococcus pneumoniae
• B. Candida albicans
• C. Escherichia coli
• D. Giardia

Answer: (D)

Secretory IgA lines mucosal surfaces and prevents pathogens from sticking to and invading the intestinal lining. In selective IgA deficiency, this critical gut defense is lost, making infections that rely on mucosal adherence more likely. Giardia lamblia is a common protozoan that colonizes the small intestine by attaching to enterocytes. Without IgA, Giardia can adhere and persist in the lumen, leading to recurrent giardiasis with malabsorption and greasy stools. That mucosal vulnerability is why giardiasis is the infection most commonly seen in individuals with IgA deficiency. Other organisms listed can cause infections in various immunodeficiencies, but they are not as characteristically linked to selective IgA deficiency as Giardia.