An overview of serum antibody responses to influenza virus antigens.

Serum antibody responses after exposure to influenza virus antigens follow expected patterns for protein antigens. Induction of primary responses occurs in organized lymphoid tissues while secondary responses may occur in the periphery; in primary responses, IgM antibody is initially dominant whereas IgG antibody is dominant in secondary responses. Serum antibody responses have been ascribed to the HA, NA, M2, NP, and M1 proteins. Only the HA and NA antibodies have been shown to provide immunity in humans. Anti-HA antibody mediates neutralization and serum IgG anti-HA antibody is the dominant antibody in the lower respiratory tract. Since evidence indicates that most infections are acquired by the airborne route with deposition of virus in the lower respiratory tract, serum IgG anti-HA antibody is the primary mediator of immunity to influenza. Homotypic immunity is high for decades. Both antigenic drift and shift of the surface antigens reduce the effectiveness of antibody to the HA and NA and lead to renewed susceptibility to infection. Nevertheless, heterotypic antibody can convey substantial immunity with the degree dependent upon the extent of cross-reactivity for the infecting virus antigens. While serum anti-HA antibody is the major need for optimal immunity to influenza, a full complement of immune modalities is desirable to ensure maximum immunity.