Final answer:
Endocytosis is an active transport mechanism by which cells take in large particles through invagination of the cell membrane to form a vesicle, which requires energy and is essential for cellular functions like the immune response.
Step-by-step explanation:
Endocytosis is a critical process by which cells intake large particles, and it cannot occur simply by these particles traversing the cellular membrane due to the presence of tight junctions. Instead, cells use a type of active transport to move these large molecules, cell parts, or even whole cells into the cell. During endocytosis, the cell's plasma membrane invaginates to form a pocket that ensnares the target particle.
The pocket then pinches off to create a new vesicle within the cell, transporting the particle without compromising the continuity of the membrane. This process requires energy and is essential for functions such as the immune response where cells, like neutrophils, engulf pathogens through phagocytosis. There are also other forms of endocytosis like pinocytosis and receptor-mediated endocytosis, each with unique functions and importance in cellular operations.