A “first responder” is a person authorized and trained to provide immediate control and assistance during a situation. Situations range from motor accidents to natural disasters to terrorist attacks.
Historical first responders were firefighters, paramedics, and police officers. Many organizations have expanded this to include first aid attendants, lifeguards, and ski patrols. Each of these is usually first on scene and trained to provide immediate control and assistance. Other organizations include 911 dispatchers and public works employees.
What about security professionals? There is a public expectation that security professionals will engage to de-escalate situations, protect people and property, and arrest perpetrators. They follow the same protocol as first aid attendants, etc.: they provide immediate control and assistance, then hand off to higher levels of authority when they arrive.

Security professionals are the first link in the emergency response chain.
Security professionals are first responders.