Necrosis
Necrosis (in Greek  = Dead) is the name given to accidental death of cells and living tissue.
Necrosis is less orderly than apoptosis, which is part of programmed cell death. In contrast  apoptosis, cleanup of cell debris by phagocytes of the immune system is generally more difficult, as the disorderly death generally does not send cell signals which tell nearby phagocytes to engulf the dying cell. This lack of signalling makes it harder for the immune system  to locate and recycle dead cells which have died through necrosis than if the cell had undergone apoptosis. The release of intracellular content after cellular membrane damage is the cause of inflammation in necrosis.

# Causes
There are many causes of necrosis including prolonged exposure to injury, infection, cancer, infarction, poisons, bites from some spiders such as brown recluses and inflammation. Severe damage to one essential system in the cell leads to secondary damage to other systems, a so-called "cascade of effects". Necrosis can arise from lack of proper care to a wound site. Necrosis is accompanied by the release of special enzymes, that are stored by lysosomes, which are capable of digesting cell components or the entire cell itself. The injuries received by the cell may compromise the lysosome membrane, or may initiate an unorganized chain reaction which causes the release in enzymes. Unlike apoptosis, cells that die by necrosis may release harmful chemicals that damage other cells. In biopsy, necrosis is halted by fixation or freezing.

# Morphologic patterns
There are seven distinctive morphologic patterns of necrosis:
- Coagulative necrosis is typically seen in hypoxic environments (e.g. myocardial infarction, infarct of the spleen).  Cell outlines remain after cell death and can be observed by light microscopy.
- Liquefactive necrosis is usually associated with cellular destruction and pus formation (e.g. pneumonia). This is typical of bacterial or, sometimes, fungal infections because of their ability to stimulate an inflammatory reaction.  Curiously, ischemia (restriction of blood supply) in the brain produces liquefactive rather than coagulative necrosis.
- Gummatous necrosis is restricted to necrosis involving spirochaetal infections (e.g. syphilis).
- Haemorrhagic necrosis is due to blockage of the venous drainage of an organ or tissue (e.g. in testicular torsion).
- Caseous necrosis is a specific form of coagulation necrosis typically caused by mycobacteria (e.g. tuberculosis).
- Fat necrosis results from the action of lipases on fatty tissues (e.g. acute pancreatitis, breast tissue necrosis).
- Fibrinoid necrosis is caused by immune-mediated vascular damage.  It is marked by deposition of fibrin-like proteinaceous material in arterial walls, which appears smudgy and eosinophilic on light microscopy.

## Arachnogenic necrosis
Spider bites are cited as causing necrosis in some areas. These claims are widely disputed. In the US at least, only the bites of spiders in the genus Loxosceles or brown recluse have been proven to consistently cause necrosis. Many other spider species are claimed to cause necrosis but in most cases firm evidence is lacking, partially because the early bite is often painless and the spider species seldom identified and because a common reaction by doctors to a possible necrotic spider bite is to remove the flesh pre-emptively.
Several species of spiders possess toxins proven to cause necrosis:
- Loxosceles spiders, including the brown recluse in the midwestern United States and the Chilean recluse in South America
Spiders suspected of, but not shown to cause necrosis:
- White-tailed spiders in Australia and New Zealand 
- Hobo spider in northwestern USA
- Sac spider in United States and Australia