Necrosis and apoptosis are differentiated by their causes, with necrosis being the accidental result of interactions with pathogens and apoptosis being the product of a programmed cell mechanism. These are the two major types of cell death in living, multicellular organisms. Despite this customary dichotomy, research indicates that the distinction between these two types of cell death are not as clear as was once thought.
Necrosis results from the release of toxins from pathogens or other trauma to cells, sufficient to cause cell death. In general this cell death is not a purposeful effect of the pathogen. It is a side effect of its digestive processes. Somewhat confusingly, patches of dead tissue are identified as necroses, even when the cause of the cell death is unknown.
Apoptosis is a vital and regular part of overall organism function and is the programmed death of cells via the cells' own chemical processes. This occurs throughout the body and is necessary to make way for replacement cells as the old ones wear out. It also occurs in response to certain pathogens, particularly viruses, as a cell which destroys itself does not propagate the virus. Apoptosis begins with the destruction of the interior structures of the cell, often followed by a rupture of the membrane and absorption of its constituents by nearby cells. Sometimes, however, as in the outermost layers of the skin, the dead cells do not rupture but stay in place and continue to serve a function after death.