Since your question is under respiratory I will answer a bit differently than the other answer (which is completely true). Some white blood cells leave the blood stream and travel to others places, one of them being the lungs, to do specific jobs. They do not return to the blood stream like the white blood cells that fight infection. There are certain white blood cells found in the lungs and they are phagocytes. They basically eat up whatever they can in order to keep your respiratory system clear. Hopefully that helps a bit as well.
Phagocytosis is the cellular process of Phagocytes and Protists of engulfing solid particles by the cell membrane to form an internal phagosome, which is a food vacuole, or pteroid. Phagocytosis is a specific form of endocytosis involving the vesicular internalization of solid particles, such as bacteria, and is therefore distinct from other forms of endocytosis such as pinocytosis, the vesicular internalization of various liquids. Phagocytosis is involved in the acquisition of nutrients for some cells, and in the immune system it is a major mechanism used to remove pathogens and cell debris. Bacteria, dead tissue cells, and small mineral particles are all examples of objects that may be phagocytosed. The process is only homologous to eating at the level of single-celled organisms; in multicellular animals, the process has been adapted to eliminate debris and pathogens, as opposed to taking in fuel for cellular processes, except in the case of the Trichoplax.