From reactive species to molecular damage
Oxidative stress describes an imbalance between the production of reactive oxygen species (ROS) and the body's ability to neutralize them. ROS such as superoxide and hydrogen peroxide arise mainly from mitochondrial respiration and from immune and enzymatic activity. At controlled levels they serve as signaling molecules; in excess they react with cellular components.
When ROS overwhelm defenses, they can peroxidize membrane lipids, oxidize and misfold proteins, and damage DNA bases. Lipid peroxidation in particular can propagate as a chain reaction, generating reactive aldehydes that cause further harm.
Defense, repair, and the accumulation tipping point
Cells counter ROS with enzymatic antioxidants (superoxide dismutase, catalase, glutathione peroxidase) and small molecules such as glutathione and vitamins C and E. The transcription factor Nrf2 coordinates much of this protective response. Damaged molecules are then repaired or cleared.
Whether a cell recovers or accumulates damage depends on the balance between insult, defense, and repair capacity, which tends to weaken with age. Notably, large trials of high-dose antioxidant supplements have not consistently improved health outcomes, so more antioxidant is not automatically better. This is general educational information, not medical advice; consult a qualified professional.