Final answer:
Individuals with selective IgA deficiency are susceptible to respiratory and gastrointestinal infections due to the absence of secretory IgA, which helps to coat and protect mucous membranes. This deficiency increases the risk of infections by various pathogens.
Step-by-step explanation:
Individuals with selective IgA deficiency are susceptible to respiratory and gastrointestinal infections because IgA plays a crucial role in protecting the mucous membranes of these body parts. IgA is found in bodily fluids such as saliva, tears, breast milk, and mucus secretions. It helps to coat and protect the extensive mucosa, preventing the entry and colonization of pathogens. The absence of secretory IgA in individuals with IgA deficiency makes them more vulnerable to infections caused by various pathogens, including H. influenzae, S. pneumoniae, Moraxella catarrhalis, S. aureus, Giardia lamblia, and pathogenic strains of E. coli.