Necrosis is the disorganized breakdown of flesh in some part of the body of a multicellular organism. In an average adult human, between 50 billion and 70 billion cells die off and are replaced every day, but necrosis refers to cell death that is unprogrammed and results from atypical body conditions, such as infections, cancer, serious injury, the presence of venom, severe inflammation, and a variety of diseases. One of the most severe forms is caused by the brown recluse spider, whose bite can cause necrotic lesions up to 10 inches (25 centimeters) in diameter.
When normal cells die off in a process called apoptosis, they send special signals to the janitor cells of the body, phagocytes, which clean them up in an orderly manner. During necrosis, the breakdown is disorganized and chaotic, and phagocytes do not consume the cells in question, nor most of the internal material released from them as they perish.
Every cell has special organelles called lysosomes, which are responsible for digesting food through the controlled release of enzymes. Lysosomes contain enzymes capable of digesting all sorts of macromolecules — for example, the enzyme lipase digests lipids, the enzyme carbohydrase digests carbohydrates, and so forth. These digestive enzymes are kept isolated from the rest of the cell by a special protective membrane surrounding each lysosome. During necrosis, these membranes dissolve, releasing the digestive enzymes indiscriminately, which go on to dissolve the membranes of further lysosomes, resulting in a chain reaction of cell death.
This cellular breakdown is a horrible affliction to undergo, and it can lead to death or permanent injury if not addressed quickly. Medical professionals have come up with a variety of ways to halt the process in people before it leads to irreversible damage, including freezing, removal of the affected tissue, or exposure to a high-oxygen environment for extended periods of time. In the past, it could frequently lead to full amputations, but today, the damage can be contained as long as the affected person is treated properly.
Necrosis does not spontaneously occur in healthy people, and the number of insects capable of causing it are relatively rare. If a person gets a spider bite, chances are that he or she has absolutely nothing to worry about. If the area around the bite begins to look infected and produces pus in large quantities, however, the bite victim should get to a hospital immediately.