They act as scavengers in the body and rob electrons from other molecules to increase their stability. The particles that are robbed don't function as they should and can be toxic. There are several types of free radicals. Some of the most common have an oxygen base. For people with ITP, imagine that our platelets are cooked by it.
Free radicals build over time.
They are a natural byproduct of our metabolism and immune system functions. They are a natural component of aging. Their production is hastened by stress, pollution, fertilizers, pesticides, prescription drugs, alcohol, electromagnetic radiation, etc. Our bodies have built in controls for free radicals and ways of changing them into neutral substances. These detoxification mechanisms require specific enzymes to make them function well.