By Dr Ananya Mandal, MD
A macrophage is a type of phagocyte, which is a cell responsible for detecting, engulfing and destroying pathogens and apoptotic cells. Macrophages are produced through the differentiation of monocytes, which turn into macrophages when they leave the blood. Macrophages also play a role in alerting the immune system to the presence of invaders.
Macrophage function in detail
The term phagocytosis is formed form the Greek words "phagein" meaning to eat, "kytos" or cell and "osis" which means process. Phagocytosis is the term used to describe the engulfing and destroying of defective or microbial cells.
When inflammation occurs, monocytes undergo a series of changes to become macrophages and target cells that need eliminating. Once engulfed, cellular enzymes inside the macrophage destroy the ingested particle. Some macrophages act as scavengers, removing dead or necrotic cells while others provide host immunity by engulfing microbes.
An ingested microbe or dead cell is engulfed in what is called a phagosome, a vesicle which is formed around the microbe by the cell membrane. This then fuses with a lysosome, another specialized vesicle that contains digestive enzymes for disintegrating the contents of the phagosome. The fused lysosome and phagosome is referred to as a phagolysosome.
Most macrophages can live for several months and can kill hundreds of different bacteria before they die. In this way, macrophages provide a non-specific or innate immunity.
Another function of macrophages is to alert the immune system to microbial invasion. After ingesting a microbe, a macrophage presents a protein on its cell surface called an antigen, which signals the presence of the antigen to a corresponding T helper cell.
The antigen being displayed is attached to an MHC class II molecule, which acts as a signal to other white blood cells that the marcophage is not actually a foreign invader even though it is displaying an antigen.
On identifying an antigen, the T helper cell activates other cells of the immune system such as cytotoxic T cells to attack the infected cell.
T helper cells also stimulate the B cells of the immune system to secrete antibodies. Each antigen has specific antibodies that are produced against it in large amounts. This "signature" antigen is also remembered by antibodies, which directly target any cells displaying the antigen in the future, should another infection occur. This developed immunity is termed adaptive or acquired immunity.
Reviewed by Sally Robertson, BSc