Apoptosis and necrosis refer to two different types of cellular death in living organisms. The main difference between these two processes is how they originate. Cells undergoing apoptosis, sometimes referred to as cellular suicide, or programmed cell death, are undergoing a natural process initiated by the cell itself to safely rid the body of old cells. When cells die via necrosis, the catalyst for death is not the cell itself but some outside factor that alters the normal physiological state of the organism and causes the cells to perish. There are several other observable differences between these two processes including the number of cells involved and the impact that each process of cellular death has on the overall health of the organism.
When the body of a living organism needs to rid itself of unwanted cells, the cell initiates apoptosis. This occurs when the cells are dangerous or they lose their function. For example, apoptosis is the natural programmed process that occurs inside the cells of a tadpole tail as the tadpole becomes an adult frog and there is no need for the tail, they are programmed to die and this results in the loss of the tail. When cells are dangerous — such as mutant cells that have the potential to turn cancerous — they also initiate apoptosis, which makes this natural process an integral part of preventing cancer.
Necrosis, on the other hand, occurs as the result damage to the cell caused by some outside factor such as disease or trauma. For example, necrosis to healthy cells may be caused by the bite of dangerous spiders such as the brown recluse, or result from the presence of a virus. This process results in the premature death of otherwise useful and healthy cells.
Another difference between apoptosis and necrosis is the number of cells involved. Apoptosis often involves the death of only one unhealthy or useless cell. Necrosis, however, generally involves the death of many cells, as the catalyst for the process — trauma or disease — typically damages more than one cell.
One of the biggest ways in which apoptosis and necrosis are different is in the impact they have on the overall health of the organism. Dangerous cellular contents that die via apoptosis are generally eliminated by other, healthy cells, which prevents the harmful contents from affecting the rest of the organism. The body does not have a way to remove the cellular contents when a cell dies via necrosis. This means that other tissues of the organism can be affected by the cellular content, which generally initiates an inflammatory response that triggers the immune system.