An outbreak of infection with a seasonal influenza A strain is reported in a small community. The strain is similar to the one that circulated in the preceding year. Approximately 70% of the adult population in that community remains uninfected, despite prolonged and repeated exposure to ill contacts. Which of the following is the most important host factor in preventing influenza infection in these subjects?

Question

An outbreak of infection with a seasonal influenza A strain is reported in a small community.  The strain is similar to the one that circulated in the preceding year.  Approximately 70% of the adult population in that community remains uninfected, despite prolonged and repeated exposure to ill contacts.  Which of the following is the most important host factor in preventing influenza infection in these subjects?

Quizplus · Accepted Answer

In this question, 70% of the adult population in a small community with an influenza A outbreak are uninfected, because they are protected either through vaccination or from prior influenza A infection (with a similar strain). A humoral response with antibodies directed against hemagglutinin is generally the most important source of protection. These antibodies can neutralize the virus and primarily block its binding to the host cells. Antibodies to neuraminidase are not the main source of protection against reinfection although they have some protective effect (decrease extent of viral invasion and shedding) (Choice B). (Choices C and D) Circulating antibodies to internal influenza virus proteins (including the viral RNA-dependent RNA polymerase) do not confer immunity. Innate immunity (eg, antigen-nonspecific macrophage responses) does not appear to prevent primary influenza infection or reinfection. Reinfection is best prevented by adaptive immune mechanisms, which are by definition antigen-specific. (Choice E) Cytotoxic CD8+ T cells specific for the major histocompatibility complex (MHC) class I-associated peptides derived from nucleocapsid proteins can kill infected cells and limit disease severity but are not thought to prevent infection. Expression of nucleocapsid proteins on MHC class II stimulates a CD4+ Th1-type T cell response (which stimulates CD8+ cell production through cytokines) and a CD4+ Th2-type T cell response. The Th2-type T cell response generates anti-nucleocapsid antibodies that do not appear to play a role in protective immunity. Educational objective: Major adaptive immune mechanisms that prevent reinfection with the influenza virus include anti-hemagglutinin antibodies.

An Outbreak of Infection with a Seasonal Influenza a Strain