CD4 and infection: Then, viral particles can overcome the epithelial barrier by: (b) direct access to the lamina propria across gaps in the epithelium, (c) capture and transepithelial transport of virions by Langerhans cells, (d) partial penetration of the epithelium and infection of intraepithelial CD4+ T cells (or other leukocytes) that then migrate to the lamina propria, or (e) epithelial crossing through intercellular spaces or by transcytosis.