The difference between a reflex and a reaction is that a reflex is an involuntary response to stimuli, while a reaction is a voluntary, intentional response that requires thought.
The University of Rochester medical dictionary describes a reflex as an automatic response that not only safeguards the body but supports its function though subconscious actions ranging from the pumping of the heart to maintaining balance while standing. The simplest reflex is termed "withdrawal" and prompts an individual to pull away from hot stove or a threatening projectile. From a physiological perspective, the action occurs as the result of a sensory neuron traveling along a nerve path, most often to the spine, where it is intercepted by a motor neuron that triggers an immediate response.
A reaction entails thought: a conscious decision to respond in a specific way. Reflex would leave an individual shivering after escaping from a frozen pond. However, it would be a reaction to dry off with a towel and seek out a heater to get warm. The physical process, like the reaction itself, is more complex and requires millions of neurons to travel to the brain and carry back the brain's instructions. A reflex is an instantaneous, instinctive action. A reaction is more measured and intentional.