Cell injury occurs when cells are exposed to stressors that overwhelm their adaptive capacities. This can lead to reversible or irreversible damage. Causes of Cell Injury: Hypoxia: Lack of oxygen, often due to ischemia (reduced blood flow). Toxins: Chemicals, pollutants, or endogenous substances that damage cells. Infections: Pathogens like bacteria, viruses, fungi, and parasites. Immunologic Reactions: Autoimmune diseases or hypersensitivity reactions. Genetic Defects: Inherited disorders affecting cellular structure or function. Physical Agents: Trauma, extreme temperatures, radiation. Nutritional Imbalances: Deficiencies or excesses of essential nutrients. Mechanisms of Cell Injury: Cell injury often involves disruptions in key cellular components: ATP Depletion: Reduced energy production impairs essential cellular functions. Mitochondrial Damage: Affects energy production and can trigger apoptosis (programmed cell death). Calcium Influx: Elevated intracellular calcium activates enzymes that damage cellular components. Reactive Oxygen Species (ROS): Free radicals cause oxidative stress, damaging lipids, proteins, and DNA. Membrane Damage: Loss of membrane integrity leads to leakage of cellular contents. Protein Misfolding and DNA Damage: Impairs cellular function and can lead to cell death. Types of Cell Injury: Reversible Injury: If the stress is removed, cells can recover. Characterized by cellular swelling and fatty changes. Irreversible Injury: If the stress is severe or prolonged, cells undergo necrosis (uncontrolled cell death) or apoptosis* (programmed cell death). Necrosis typically involves inflammation and results from external injury. Common patterns include coagulative necrosis (e.g., in myocardial infarction), liquefactive necrosis (e.g., in brain infarcts), and caseous necrosis (e.g., in tuberculosis). Apoptosis is a regulated process that eliminates unwanted or damaged cells without causing inflammation. It plays roles in development and tissue homeostasis.