Are reactive oxygen species (ROS) good or bad?

Reactive oxygen species (ROS) may be either harmful or beneficial based on the circumstances and the concentration. When maintained at proper levels, ROS has important roles in cellular signaling, in the immune system, and in regulation of gene expression. A lack of ROS in the immune system may impair the body to fight pathogens. Neutrophils and macrophages in particular produce ROS as part of the immune response to neutralize invading pathogens. Additionally, ROS can act as signaling molecules in cellular pathways. ROS can modulate the activity of signaling molecules and transcription factors, influencing gene expression and cellular behavior. On the other hand, a build up of excessive ROS in cells may cause damage to DNA, RNA, and proteins, ultimately leading to cell death. This process is known as oxidative stress. Oxidative stress can cause cell dysfunction and contribute to the development of aging processes, cancer, and neurodegenerative diseases. Excessive ROS can also cause damage to nearby tissue, leading to chronic inflammation, oxidative stress, and tissue injury in adjacent cells.