Macrophages are a type of white blood cell which eat foreign material in the body. These cells are involved in the primary or innate immune response to a number of immune invaders, and they also make up an important part of the body's acquired immune system. At any given time, these cells are at work in many corners of the body, quietly cleaning up foreign debris, bacteria, and viruses before they have a chance to cause a problem.
Like other blood cells, these blood cells start out in the bone marrow. The life cycle of these cells actually starts with a type of cell called a monocyte, which has the capability to mature into a macrophage when it is stimulated to do so. Some monocytes drift to specific areas of the body, such as the liver, where they mature into specialized macrophages which remain in place, while others turn into free-floating macrophages.
In a sense, these cells are like security guards for the immune system. Some of them remain stationed at their regular "desks" near areas where foreign material commonly enters the body, routinely screening the materials which pass them to look for things which could be dangerous. Others roam on patrol, looking for intruders who may have slipped past other guards.
When a macrophage encounters something which it thinks might be dangerous, it will engulf it and create enzymes to neutralize it so that it cannot continue replicating in the body. This process is called phagocytosis, literally "eating cells." Macrophages use phagocytosis to collect antigens which they can present to helper T-cells, alerting the T-cells to the fact that there is a foreign invader in the body, and triggering an immune response.
Scientists still have some learning to do with these unique cells. For example, originally researchers thought these cells damaged the DNA of their victims to prevent them from replicating, but studies published in 2009 showed that the enzymes these cells generate actually work differently. More study can provide detailed information about how the body responds to infection, and the ways in which immune cells can go wrong.
These scavenger cells can sometimes cause problems in the body. These cells have been implicated in the development of lesions such as granulomas, caused by chronic inflammation. These cells play a role in inflammatory processes, so when they become overactive, they can actually cause harm, rather than simply protecting the body from something foreign. Some types of cancer also appear to be aggravated by macrophages, and these cells can be hijacked by the HIV virus and used to spread it in the body.