White blood cells are a collection of different immune cell types that respond to inflammation and directly attack invading pathogens during an innate immune response.
The key cellular players of the immune response are the leukocytes, or white blood cells, that circulate in the blood and lymph and search through tissues looking for any signs of pathogens or foreign particles. They are actively drawn toward pathogens during an inflammatory response through the process of chemotaxis, the movement of a cell in response to a chemical stimulus. Once they arrive at the site of infection, they have several tools that enable them to fight parasites, the most prominent of which is phagocytosis.
Phagocytosis is the process by which a leukocyte engulfs and destroys its target through a series of steps. The first step is contact, where the leukocyte uses its cell surface receptors to bind to the target, leading to the subsequent steps of phagocytosis. The process of phagocytosis is aided by antibodies and the complement system, which opsonize the target. The second step is engulfment, where the leukocyte changes the shape of its cell membrane to produce extensions in order to surround the target and engulf it entirely. Once the target is fully engulfed, it is internalized in a vesicle, a membrane sac formed from the plasma membrane, called a phagosome in the third step. In the fourth step, the target-containing phagosome then fuses with another type of vesicle in the cell, called a lysosome, an organelle in the cytoplasm of eukaryotic cells containing degradative enzymes enclosed in a membrane. The fusion of the two vesicles, phagosome and lysosome, results in the formation of a phagolysosome. In the final step, the target is completely degraded and killed using the contents of the lysosome.