Final answer:
The initial compartment a pathogen enters after being engulfed by a phagocyte is the phagosome. This vesicle then fuses with a lysosome to form a phagolysosome, where the pathogen is destroyed.
Step-by-step explanation:
The initial compartment that a pathogen enters after being engulfed by a phagocyte is known as a phagosome. When the phagocyte, such as a macrophage, encounters a pathogen like a bacterium, it extends parts of its membrane to envelop the bacterium and internalizes it into a phagosome. This vesicle is an isolated compartment within the cytoplasm of the phagocyte.
The next step involves the fusion of the phagosome with a lysosome to form a phagolysosome, where powerful digestive enzymes and acids work to kill and degrade the pathogen. Although some pathogens have developed mechanisms to evade this process, the fusion of the phagosome and lysosome is a crucial aspect of the phagocyte's ability to destroy pathogens.