Your cells die every day. Don’t worry, your body is protecting itself. In a process known as apoptosis or programmed cell death, cells that are no longer needed commit suicide. Some cells are only required for a short time, they may be infected by a virus or develop harmful cancerous mutations. Cell death is also an essential part of development from an embryo. For example, mouse paws begin as spade-like structures and only form the individual digits as the cells in between die. During apoptosis the cells fragment into smaller apoptotic bodies, and their cell surface is flipped open to display lipid molecules called phosphatidylserines, which act as an ‘eat me’ signal to recruit cells called macrophages to engulf them, before their contents spill out and damage the surrounding tissue. This is a process known as efferocytosis.
However cell death is not always so orderly. Some cells suffer premature death known as necrosis, where they burst open for various reasons such as infection, physical trauma or extreme temperatures. As the cell’s contents are released into the open, an inflammatory response is triggered. Studies are ongoing to establish the many mechanisms by which this process occurs. For example, it is known that proteins on the surface of immune cells, called pattern recognition receptors or PRRs, detect the material released by dying cells, and trigger the expression of genes leading to the release of inflammatory substances. When an infection occurs, the immune response is usually short-lived as it rapidly kills the infection. However, when cell necrosis and subsequent inflammation occurs over a longer period, the substances released by macrophages – that have migrated in to engulf the dying cells – can damage the surrounding tissue, resulting in a build-up of dead cells.