CD9 and infection: Compelling evidence indicates that the large extracellular loop (LEL) of mast cell-expressed TET proteins [Cluster of differentiation (CDs), such as CD9, CD63, CD81, CD82, CD151] plays a key role in the route of pathogens infection (Pileri et al., 1998; Ho et al., 2006; Mazurov et al., 2007; Shanmukhappa et al., 2007; Singethan and Schneider-Schaulies, 2008).