The B subunit is responsible for the binding of the holotoxin to specific receptors on the target cell surface, such as globotriaosylceramide (Gb3) in human intestinal microvilli. Shiga toxin contains a single subunit A and five copies of subunit B. There are three Gb3-binding sites in each subunit B monomer, allowing for a tighter binding to the target cell. Binding sites 1 and 2 have higher binding affinities than site 3 (By similarity). Belongs to the StxB family.