Final answer:
Selective IgA deficiency makes individuals prone to respiratory and gastrointestinal infections due to the lack of secretory IgA, a key defense mechanism in these areas.
Step-by-step explanation:
Individuals with selective IgA deficiency are susceptible to respiratory and gastrointestinal infections because they are not able to produce secretory IgA, which is normally an important defense mechanism in these areas. Infections in the lungs and gastrointestinal tract can involve various pathogens, including H. influenzae, S. pneumoniae, Moraxella catarrhalis, S. aureus, Giardia lamblia, or pathogenic strains of Escherichia coli.