The macrophage is a vital type of white blood cell. A macrophage is a kind of swallowing cell, which means it functions by literally swallowing up other particles or smaller cells.
Macrophages engulf and digest debris (like dead cells) and foreign particles through the process of phagocytosis, so macrophages act like scavengers. They are constantly roaming around, searching for and destroying dead cells and foreign particles that don’t belong in the body. Because they cannot identify specific targets, macrophages are considered part of the innate immune response. Macrophages also directly aid the specific immune response.
Some macrophages concentrate near the lining of the intestine, which is a natural point of entry to many outsiders due to its absorbant nature.
Macrophages come from specific white blood cells called monocytes. Monocytes are born from stem cells in the bone marrow and circulate throughout the blood stream. Once a monocyte leaves the blood, it matures into a wandering macrophage or a fixed macrophage. Wandering macrophages travel throughout both blood and lymph streams to perform their job; fixed macrophages strategically concentrate in specific areas that are more vulnerable to intruders like the lungs or the intestine. Macrophages can then be found in many areas in the body, like different tissues, lungs, skin, and also organs of the immune system like the spleen, lymph nodes, and bone marrow.
Macrophages are programmed to look for and eat any foreign particles that live in the fibrous environment (extra cellular matrix) between cells, as well as eat the debris of damaged or dead cells. Special receptors sites on the cell membrane enable the macrophage to receive chemical signals sent out by bacteria, attracting them to points of infection.
Macrophages distinguish between body cells and outsiders by recognizing the specific structure of proteins that coat healthy body cells. This is one way in which the innate immune system is able to differentiate between self and nonselfs, so that macrophages don’t attack healthy cells!
Macrophages can activate the acquired immune system!
One of the most important functions of the macrophages is that they can activate the acquired immune system! After a macrophage has eaten and digested a particle, it displays some of the broken down germ proteins (antigens) on its cell surface. These antigens act as identification signals for Helper T cells. Helper T cells can “read” these signals and tell what kind of particle the macrophage has eaten! If the T cell determines the macrophage has eaten something harmful (a pathogen), it can trigger a powerful reaction towards the specific pathogen.
When a macrophage encounters an outsider, it extends its cell membrane around the particle, drawing the particle into itself. It then forms a vesicle called a phagosome. Lysosomes inside of the macrophage release enzymes that break apart the captured particle inside of the phagosome.
Macrophages are not the only types of cells that function through phagocytosis. There are other important swallowing cells that make up the immune system, such as cells called granulocytes, neutrophils and dendritic cells. Macrophages are the biggest and most effective of the phagocytes.
Phagocytosis was one of the earliest form of metabolism, meaning that it was initially used simply for getting food for energy. You can still see some organisms using phagocytosis for eating and digestion, like the amoeba from the Protista kingdom. The amoeba is a single-celled organism that can swallow other cells, then break down the cell and use it for energy—the same way that macrophages swallow and break down bacteria and harmful pathogens. This ancient swallowing ability has been retained by some cells in modern organisms, like the macrophages, for protection!
So, phagocytosis has evolved from a simple form of metabolism to being also a major part of our immune system, helping to keep us healthy!