Your brain focuses on some alleged thread, for instance, a very scary thought that was floating somewhere at your subconscious.  Your thalamus – the part of the brain responsible for regulating consciousness, sleep and alertness – transfers that information to your amygdala – the part of the brain responsible for emotional reactions, decision-making and memory – which marks it as “danger” and sends a signal to your sympathetic nervous system, activating the fight-or-flight response.
Anxiety disorders are characterized by a general feature of excessive fear (i.e. emotional response to perceived or real threat) and/or anxiety (i.e. worrying about a future threat) and can have negative behavioral and emotional consequences. Obsessive-compulsive and related disorders are characterized by obsessive, intrusive thoughts (e.g., constantly worrying about staying clean, or about one's body size) that trigger related, compulsive behaviors (e.g. repeated hand-washing, or excessive exercise). These behaviors are performed to alleviate the anxiety associated with the obsessive thoughts. Trauma- and stressor- related anxiety disorders are related to the experience of a trauma (e.g., unexpected death of a loved one, a car accident, or a violent incident) or stressor (e.g., divorce, beginning college, moving).
There is a long list of signs and symptoms of an anxiety attack. But because each body is somewhat chemically unique, anxiety attacks can affect each person differently. Consequently, anxiety attack symptoms can vary from person to person in type or kind, number, intensity, duration, and frequency. If your symptoms don’t exactly match this list, that doesn’t mean you don’t have anxiety attacks. It simply means that your body is responding to them slightly differently.