What is the process by which a cell engulfs large particles as defense or for food

Phagocytosis.

Explanation:

Phagocytosis is the process by which a cell engulfs large particle as defense or food.

In the process of phagocytosis the cell extends its plasma membrane traps the particle or antigen into a phagosome.

Inside the phagosome several lytic enzyme act on the particle and thus digests it.

Macrophages in our body are good exmples of phagocytic cells.

Unicellular organism like Amoeba also carries phagocytosis for food.