CD79A and infection: In contrast, in calves with high levels of colostrum-derived serum antibody titers, BRSV IgA in the upper respiratory tract is undetectable in the period immediately following BRSV infection or vaccination [15]; however, despite the absence of an immediate upper respiratory BRSV IgA response following vaccination, anamnestic nasal IgA responses have been reported as early as 6 days following BRSV experimental infection [10,15].