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Cells can suffer damage resulting from infection, trauma, or injury.

When this happens, cells must decide how to respond for the good of the body. In this lesson, we’ll learn how a cell recognizes injury and uses apoptosis, necrosis, and other signals to die or even kill itself.

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How Injuries Are Recognized

This may seem grim, but let’s face it, the two main ways someone can die is by their own hand or someone else’s.

The term ‘someone else’s hand’ doesn’t necessarily mean a human’s hand; it could mean something like a disease or trauma. If you experienced trauma, you’d be able to instantly recognize the signs and symptoms of an injury, such as pain and bleeding and so on. Cells have a similar talent.

This lesson is all about injuries and how cells respond to them. We’ll go through a general overview of how cells recognize that they are injured and end up dying as a result of the injury.

What Is Apoptosis and Necrosis?

In some cultures around the world, suicide isn’t as much of a taboo subject as it is in the West. In fact, long ago in some cultures, elder members of society would go off and kill themselves or ask to be killed in times of great famine in order to make sure enough food was around for the younger generations. This is quite gruesome to many of us, but the cells in your body many times sacrifice themselves individually for the benefit of society at large. In our case, the society is the entire collection of cells that make up your body.

This suicide, or programmed cell death, is known as apoptosis. It is quite literally an almost conscious decision, if you will, by a cell to kill itself for any number of reasons, including during the normal growth and differentiation of human embryos, not just due to some kind of damage. This is in contrast to necrosis, which is the premature death of cells due to a traumatic event from something like blunt force trauma, toxins, drugs, temperature extremes, radiation, and so forth. Necrosis usually affects many more cells simultaneously, as opposed to apoptosis, which typically affects individual cells separately.


In the case of necrosis, some kind of trauma causes the damaged and dying cells to swell and break apart. As a cell is swelling, its cell membrane breaks apart and becomes more porous. Some of the substances released during this breakdown, such as free fatty acids, act as a signal to neighboring cells. These signals land on the cell surface receptors of neighboring cells and almost act as a doorbell of sorts.Just like if you were to hear a doorbell ring, you’d recognize someone was at the door, so too will these neighbors recognize the neighbor is dying.

This signal causes the neighboring cells to begin to become quite concerned about the fact that all this damage is occurring nearby. The neighboring cells signal the immune system to get over to the area as quick as possible and initiate an inflammatory response against whatever it is that is causing the injury.In essence, when it comes to necrosis, neighbors call 911 to get the police to respond as quickly as possible to the scene of a crime. In our case, that scene is called necrosis. This can be a good thing. If an invader, such as bacteria, is causing this cell death, then the dying cells scream out, the neighbors call the police, and the police shoot the bacteria, thereby preventing more cells from getting infected or killed.

The problem is that long-term inflammation may actually damage healthy tissue in addition to any pathogen that may be invading the body. Therefore, necrosis is kind of a messy deal.


In contrast to necrosis, apoptosis is an internal decision by a cell to kill itself that isn’t nearly as messy. As an illustration of apoptosis, if a cell’s DNA is damaged beyond repair, it may use something known as p53, a tumor suppressor protein that will initiate a sequence of events that signal the cell to commit suicide. This is done for good reason because if there’s enough DNA damage, then the cell may turn into a malignant cell that multiplies without control, resulting in a malignant tumor.

Therefore, this one cell sacrifices itself to prevent the formation of cancer, which would damage the rest of your body quite severely.Basically, unlike necrosis, the cell dies during apoptosis in an orderly and controlled fashion that occurs as a result of some kind of justifiable need to kill the cell, such as the prevention of cancer formation. This death causes the cell to shrink, as opposed to swelling of the cell seen during necrosis, and does not cause a potentially damaging inflammatory response.Now, as a quick recap, during necrosis, molecules are released by dead and dying cells that cause the inflammatory response to be triggered. This inflammatory response can be triggered by neighboring cells as well that recognize the injury or by the molecules flying off to distant sites to cause an inflammatory response to occur.

Then, inflammatory cells, such as white blood cells, come into the area to kill off any pathogen and clean up any messy debris that came about as a result of all that death and destruction. In apoptosis, a single cell dies kind of quietly, with no inflammation, and is cleaned up by white blood cells or neighboring cells.There’s one other way by which an injured cell can be recognized and killed. In this case, a cell may not want to or be able to kill itself as per apoptosis and has not undergone necrosis. Instead, it will signal a white blood cell that it wants to be killed.

One reason for this in real life may be a viral infection.In some cases, a virus will enter a cell and just hide there. The cell, not able to kill itself and not damaged enough by the virus to undergo necrosis, will instead use a little molecule on its cell surface to signal to a white blood cell that there is a virus hiding inside. The white blood cell then recognizes this danger and releases chemicals that end up killing the cell harboring the virus inside. This prevents the virus from multiplying and infecting other cells in your body.

Lesson Summary

So, when a cell recognizes that it’s injured, it can kill itself using programmed cell death, also known as apoptosis.

As an illustration of apoptosis, if a cell’s DNA is damaged beyond repair it may use something known as p53, a tumor suppressor protein that will initiate a sequence of events that signal the cell to kill itself in order to prevent further damage to the body.This is in contrast to necrosis, which is the premature death of cells due to a traumatic event from something like blunt force trauma, toxins, drugs, temperature extremes, radiation, and so forth. Here, the cell will release molecules during its destruction that signal other cells that it is dying, thus causing an inflammatory response to occur.Finally, other times a cell doesn’t get injured enough to undergo necrosis and cannot use apoptosis to kill itself.

In this case, it may try to use a molecule on its cell surface to signal a white blood cell to kill it as a result of some kind of injury or infection.

Learning Outcomes

Once the lesson is over, you could be able to:

  • Explain how cells get word of an injury
  • Relate the process of necrosis and the ways in which it contacts other cells
  • Evaluate apoptosis as a form of cellular self-destruction

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