Ultrastructure of B cells infected with S. typhimurium (ST) and stimulated with phorbol 12-myristate 3-acetate (PMA). a-b) Control B cells. c) PMA-stimulated B cell, which has abundant vacuoles of different sizes. d) The field magnification of a PMA-stimulated B cell (circle) shows macropinosome formation (black narrow) and the presence of macropinosomes that are already formed in various sizes (arrowheads). e) Micrograph of S. typhimurium-infected B cell, which shows that the bacillus is surrounded by large membrane extensions (narrow). f) S. typhimurium-infected B cell with internalised bacteria (arrowheads), thin narrows depicts a multilamellar structure (left) and a late degradative autophagic vacuole (LDAV) (right).