Final answer:
Selective IgA deficiency is associated with an increased risk of respiratory and gastrointestinal infections due to the absence of secretory IgA, which is necessary for protecting mucosal surfaces from pathogens.
Step-by-step explanation:
Selective IgA deficiency is clinically associated with increased susceptibility to respiratory and gastrointestinal infections due to a lack of secretory IgA which is crucial for mucosal immunity.
Selective IgA deficiency is an immunodeficiency where individuals cannot produce secretory IgA, which is an antibody that plays a critical role in protecting mucosal surfaces. Normally, IgA coats and safeguards the mucosa present in the gastrointestinal and respiratory tracts, among others, totaling approximately 4000 square feet in humans. In individuals with this deficiency, although they produce normal levels of IgG and IgM antibodies, the absence of IgA leaves these mucosal surfaces vulnerable to colonization and invasion by pathogens.
This results in a greater susceptibility to infections by bacteria such as Haemophilus influenzae, Streptococcus pneumoniae, and Giardia lamblia, since IgA is essential in neutralizing these infectious agents. Its absence means that the local humoral response at epithelial surfaces is compromised, which normally would prevent infection by binding and neutralizing pathogens. Therefore, people with selective IgA deficiency experience recurrent infections in these areas, where secretory IgA would typically provide a first line of immune defense.