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Summary of Cell Injury :-

Cell damage (also known as cell injury) is a variety of changes of stress that a cell suffers due to external as well as internal environmental changes. Amongst other causes, this can be due to physical, chemical, infectious, biological, nutritional or immunological factors. Cell damage can be reversible or irreversible. Depending on the extent of injury, the cellular response may be adaptive and where possible, homeostasis is restored. Cell death occurs when the severity of the injury exceeds the cell's ability to repair itself. Cell death is relative to both the length of exposure to a harmful stimulus and the severity of the damage caused. Cell death may occur by necrosis or apoptosis.

Causes
  1. Physical agents such as heat or radiation can damage a cell by literally cooking or coagulating their contents.
  2. Impaired nutrient supply, such as lack of oxygen or glucose, or impaired production of adenosine triphosphate (ATP) may deprive the cell of essential materials needed to survive.
  3. Metabolic: Hypoxia and Ischemia
  4. Chemical Agents
  5. Microbial Agents:-Virus & Bacteria
  6. Immunologic Agents: Allergy and autoimmune diseases such as Parkinson's and Alzheimer's disease.
  7. Genetic factors: Such as Down's syndrome and sickle cell anemia 

Targets
The most notable components of the cell that are targets of cell damage are the DNA and the cell membrane.
  1. DNA damage: In human cells, both normal metabolic activities and environmental factors such as ultraviolet light and other radiations can cause DNA damage, resulting in as many as one million individual molecular lesions per cell per day.
  2. Membrane damage: Damage to the cell membrane disturbs the state of cell electrolytes, e.g. calcium, which when constantly increased, induces apoptosis.
  3. Mitochondrial damage: May occur due to ATP decrease or change in mitochondrial permeability.
  4. Ribosome damage: Damage to ribosomal and cellular proteins such as protein misfolding, leading to apoptotic enzyme activation.

Apoptosis 
Apoptosis is the programmed cell death of superfluous or potentially harmful cells in the body. It is an energy-dependent process mediated by proteolytic enzymes called caspases, which trigger cell death through the cleaving of specific proteins in the cytoplasm and nucleus. The dying cells shrink and condense into apoptotic bodies. The cell surface is altered so as to display properties that lead to rapid phagocytosis by macrophages or neighbouring cells. Unlike necrotic cell death, Neighbouring cells are not damaged by apoptosis as cytosolic products are safely isolated by membranes prior to undergoing phagocytosis. It is considered an important component of various bioprocesses including cell turnover, hormone-dependent atrophy, proper development and functioning of the immune and embryonic system, it also helps in chemical-induced cell death which is genetically mediated. There is some evidence that certain symptoms of "apoptosis" such as endonuclease activation can be spuriously induced without engaging a genetic cascade. It is also becoming clear that mitosis and apoptosis are toggled or linked in some way and that the balance achieved depends on signals received from appropriate growth or survival factors.