Receptor-mediated endocytosis allows cells to internalize relatively small extracellular molecules that bind to the cell’s receptors. Certain regions of the cell membrane are lined with a protein called clathrin and studded with receptors. These regions are internalized to form an endocytic vesicle, bringing with them the receptors along with their bound ligands. In contrast, phagocytosis allows a cell to bring in relatively large particles, including bacteria. The cells send out arm-like extensions, pseudopods, which surround and enclose extracellular material. This action envelopes the material, bringing it into the cell in an enclosed compartment called a phagosome.