Detection of a-2,3and a-2,6-linked sialic acid receptors on progenitor epithelial cells Influenza virus infects cells through binding to cell surface sialic acid receptors. To examine the susceptibility of progenitor epithelial cells to influenza virus, we first evaluated by flowcytometry these cells for the presence of sialic acid receptors. The a-2,3-and a-2,6-linked sialic acid receptors were detected on the surface of progenitor epithelial cells by lectin staining. A majority of progenitor cells (.95%) expressed both a-2,3and a-2,6-linked sialic acid receptors, indicating that viruses of both avian and mammalian lineages might replicate in these cells (Fig. 3A ).