A phagosome is a membrane-bound vesicle formed within a cell that engulfs large particles, such as pathogens or debris, during the process of phagocytosis. This vesicle then fuses with lysosomes to form a phagolysosome, where the engulfed material is broken down and digested. The formation of a phagosome is a crucial part of the immune response and helps maintain cellular homeostasis.