Apoptosis, or programmed cell death, is a mechanism for organisms to eliminate cells that have been damaged or altered, thereby allowing organisms to maintain their genetic integrity. Apoptosis is a natural and necessary process that is also crucial for cell turnover, immune system development, and embryonic development. It is generally beneficial, as opposed to necrosis. Necrosis, which is caused by external factors, is always detrimental and at times can be fatal.