CAT and Alzheimer disease: Both free radicals and reactive oxygen species (ROS) “contribute” to the process of AD, such as increasing damage to synaptic plasticity and triggering neuroinflammation, but small amounts of ROS are irrelevant to AD, mainly because of the presence of oxidative stress defense systems consisting of antioxidant enzymes that can scavenge them, such as superoxide dismutase (SOD) and catalase.