The p53 gene (marked in red in the above picture) in our cells puts a stop to cell division and triggers off apoptosis (the process of programmed cell death (PCD))
when the DNA of cells are severely damaged or infected with germs and viruses
Sending a “suicide” signal (the p53 gene) to uncompetitive cells
Cell competition exists among cells. When genetic variations occur for some reason and thus decelerate some cells’ division processes, the slow-dividing cells are prodded by nearby cells into committing suicide. In the process, innate immunity kicks in.
However, when genes related to innate immunity malfunction, slow-dividing cells do not kill themselves through p53-induced apoptosis and go on living.
The innate immune system refers to the process of macrophages ingesting pathogens or infected cells killing themselves to defend the host from infection by other organisms. Antibodies are produced in the adaptive immune system. The phenomenon of cell competition was found in the 1970s for the first time. A drosophila (fruit fly) which has a flawed a ribosomal protein gene or problems in the genes that facilitate cell division still goes on living, growing slowly.
However, in most cases, drosophila embryos containing both normal and damaged cells grow into healthy drosophilae comprised of normal cells. A closer look at the process shows that perky, normal cells kill and eat damaged cells to expedite their cell division processes. In other words, damaged cells were beaten out by strong ones in cell competition. Still, little is known on how healthy cells detect the existence of unhealthy cells and send “death” signals to them.
The obvious thing is that if cells that should cease to exist went on living, they would turn into cancerous cells. Defender cells T and B cells are released from the thymus and the bone marrow, respectively. As a matter of fact, T cells originate from T stem cells (precursor cells) in the bone marrow. T cells serve as consumables in our body.
When healthy stem cells move to the thymus from the bone-marrow, they beat out the preexisting somewhat withered T stem cells in the thymus and then divide into T cells.
When T stem cells are unable to move to the thymus from the bone-marrow, some of the old stem cells withering in the thymus morph into cancerous cells, consequently causing leukemia.
The reason why T stem cells’ prolonged stay in the thymus transforms them into cancerous cells is still unclear. Noteworthy is that in cell competition, apoptosis occurs when unhealthy cells are put together with their healthy counterparts. However, when they are separated from the healthy cells, they go on living, albeit in a lethargic manner, raising their chances of becoming cancerous cells.