Fig. 6. Thin sections through a portion of the plasma membrane of three U937 macrophages. All are printed at the same magnification. All three sections show ER vesicles attached to the plasma membrane. In the areas indicated by the arrows we can see the hairs that attach these membranes together. (a) This macrophage was exposed to L. pneumophila for 15 minutes at an MOI of 20 before fixation. Extracellular L. pneumophila were present near this section as well as internalized L. pneumophila. (b) and (c) Uninfected U937 cells. As in (a). ER vesicles are attached to the plasma membrane.