The Upper Respiratory Tract of Many Individuals Is Colonized by Streptococcus

The upper respiratory tract of many individuals is colonized by Streptococcus pneumoniae bacteria. Infections caused by these bacteria, including pneumonia, meningitis, otitis media (ear infections) , and sinusitis, are thought to occur in individuals lacking protective antibodies. For many years, IgG was thought to be the major antibody class responsible for protective immunity to S. pneumoniae, due to the ability of anti-bacterial capsule IgG antibodies to opsonize the bacteria and promote phagocytosis. However, in addition to IgG, pneumococcal polysaccharides elicit robust IgA antibody responses. It was traditionally thought that these IgA antibodies functioned in neutralization, by blocking bacterial attachment to mucosal epithelial cells. It is now known that IgA antibodies, like IgG, can function as opsonins, to induce phagocytosis and killing of IgA-coated pathogens. This function of IgA antibodies depends on:

A) The production of high affinity anti-bacterial IgA antibodies
B) The dimeric form of IgA antibodies to bind to multivalent sites on the bacteria
C) The ability of dimeric IgA binding to recruit and activate the complement cascade
D) The presence of IgA-specific Fc receptors on neutrophils and macrophages
E) The ability of IgA antibodies to efficiently cross the epithelial surface and enter the airways