We present a model in which animals are passively immunized at a mucosal site, allowing one to evaluate immunological protection at the mucosal level only. Affinity-purified, anti-M protein sIgA administered intranasally protected mice against systemic infection after intranasal challenge with group A streptococci. In contrast, anti-M protein serum Ig administered intranasally was not protective at this site, although it neutralized the antiphagocytic property of M protein and promoted phagocytosis. Protection by sIgA occurred despite the lower immunoreactivity of sIgA to purified M protein compared with serum Ig. The data suggest that sIgA can protect at the mucosa and may preclude the need for opsonic IgG in preventing streptococcal infection.

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