Phagocytosis or “cell eating” is a way of getting large matter inside the cell. During this process, the cells form “pseudopods” or special folds of their cell membrane and entrap or enclose food particles, small organisms, and other particulate matter. The folds then fuse and form a vacuole that pinches off from the membrane. Once inside the cytoplasm, the vacuole is now called a phagosome and it fuses with a lysosome. Lysosomal enzymes then digest the particle and release the digested material to the cytoplasm where it is used by the cell for various purposes. Phagocytosis is how the Amoeba obtains its food from the environment.
In animals, some cells act as “professional” phagocytes. Among these are the neutrophils and macrophages. These cells act like roving guards. They move around in the body eating or phagocytizing any particulate material that they encounter. The particulate material can include foreign invaders, dead or damaged cells, and cellular debris. So in these cells, phagocytosis is more of a clean up process rather than an eating process.
In the bone, a special phagocyte, the osteoclast, “eats” old cartilage and bone tissue and partners with the osteoblast, a “bone builder”, in bone formation, reconstruction and repair.
While the “eating habits” of our phagocytes are meant to protect us, sometimes, some enterprising microorganism can take advantage of this activity. Mycobacterium tuberculosis, the tuberculosis-causing bacterium for example, is one such enterprising organism. Once taken inside the cell through phagocytosis, the bacterium secretes an enzyme that prevents fusion of the phagosome with the lysosome. Thus, lysosomal enzymes cannot digest the bacterium and it then can multiply inside the cell and infect other cells too.
So, cells just like organisms should also watch what they are eating or suffer the consequences.