How do cells within cells form?

Cells can form within cells by mechanisms such as entosis, phagocytosis, and cannibalism. Entosis is a specialized mechanism of cell-in-cell formation where one cell engulfs and digests another viable cell. It occurs through a process of invagination and  cell-cell adhesion. Entosis involves the formation of an entotic vacuole within the host cell. The target cell is eventually degraded by the host cell’s lysosomes at the end of the process. Cancer cell cannibalism is defined as a mechanism in which a larger cell containing a smaller one (usually in a large cytoplasmic vacuole), is detected in tumor cells. The process starts with the attachment of cannibalistic cells to a free cell. Eventually, the engulfed cell gets interiorized within the cannibalistic cell and may either die, divide inside the vacuoles, or completely escape. Phagocytosis is a process that engulfs large particles, microorganisms, or other cells. A phagocyte engulfs a dying or apoptotic cell. The phagocyte extends pseudophakia around its target cell, creating a phagosome that holds the internalized cell. The phagosome then fuses with lysosomes to generate a phagolysosome, where the internalized cell is degraded.