A 7-year-old boy is brought to the pediatric clinic for his third episode in 18 months of watery, foul-smelling, greasy stools that float in the toilet, accompanied by abdominal bloating, cramping, and flatulence. He was diagnosed with selective IgA deficiency 3 years ago after evaluation for recurrent sinopulmonary infections; his immunoglobulin panel showed undetectable serum IgA with normal IgG and IgM. He attends a daycare center, and his mother reports he has lost 2 kg over the past 6 months. There is no blood or mucus in the stool. A stool antigen enzyme immunoassay is positive for Giardia lamblia.
Which of the following best explains why this child is predisposed to recurrent, refractory Giardia infections?