Final answer:
Individuals with selective IgA deficiency are susceptible to respiratory and gastrointestinal infections because they cannot produce secretory IgA, which acts as a barrier against pathogens. This makes them more vulnerable to various types of pathogens, including bacteria and parasites.
Step-by-step explanation:
Individuals with selective IgA deficiency are susceptible to respiratory and gastrointestinal infections because they cannot produce secretory IgA, which is an important defense mechanism in these areas. Secretory IgA acts as a barrier, coating pathogens and making it physically impossible for them to bind to receptors and cause infection. Without secretory IgA, these individuals are more vulnerable to a variety of pathogens, including bacteria like H. influenzae and S. pneumoniae, as well as parasites like Giardia lamblia.