Macrophages are immune system cells which "eat" foreign invaders and dead cells. A macrophage can be a wanderer, or it can be localized to many areas of the body that may be exposed to infection, such as the lungs. Lung macrophages scavenge the surface of the lungs for invaders that have gotten past the nose and throat barriers.
Human immunity is made up of many different cells and molecules, each with its own particular function. A subgroup of immune system cells are known as white blood cells, and macrophages are part of this group. A macrophage's job is to ingest things that it recognizes as foreign and to clean up scraps of dead host cells.
It does this through phagocytosis, which comes from the Greek words phago and cyte, which mean "eat" and "cell," respectively. The macrophage engulfs the item and, once it is inside the cell, breaks it down with enzymes into manageable molecules. Macrophages can move around, which makes it easier to get to the target item.
Lymph and blood provide avenues of travel for some macrophages. Some stay in one area, and lung macrophages are one such example. Before the lung macrophages get to the lungs, they have to be formed. The first step in a lung macrophage's life is to be formed in bone marrow as a monocyte, another type of white blood cell. Monocytes circulate in the blood, and only when they move into other tissues do they mature into macrophages.
The reason some macrophages end up in the lungs is because the lungs are one way that infectious agents such as bacteria or viruses can get into the body. The membranes and compounds of the nose, mouth, and airways can filter out some organisms, and the lung macrophages try to kill the survivors. An average person's lung has several million macrophages ready to engulf invaders, but when an infection occurs, hundreds of millions of new macrophages can arrive to help.
Inside the lungs, macrophages can either populate the surface of the lungs as lung macrophages or move into the alveoli of the lungs, where they are known as alveolar macrophages. The alveoli of the lungs are tiny compartments where the body swaps fresh, oxygenated air for waste gases, which are then exhaled. Lung macrophages can turn into alveolar macrophages.
Apart from finding and eating destroyed cells and foreign organisms, macrophages can also produce signals to tell the immune system what to do next. To do this, they display antigens from the item ingested on the outside of the cell. Other cells called Helper T-cells may recognize the antigen and then spark off an immune response specifically against that infectious agent.